There are several factors at work here. One banal example is that traditional Roman garb is juist unfit for Northern Europe. You want trousers, a wollen vest and some good boots during a Northern winter. You don't want to wear a toga and sandals. It just isn't practical. Since post-Roman Europe sees an increasing development of Northern Europe, certain changes on that front are going to be inevitable. Even if more Roman clothing stays more popular in Southern Europe, it's not going to be that way all over Europe.
As for architecture, art etc. ...One issue is the simple fact that the Roman collapse, and the increasing divergence of Western and Eastern Europe, led to a lack of availability when it came to certain products or ingedients (certain dyes, for instance, became far less easily available). Another issue is that the lack of the centralised Roman state apparatus (and the initial lack of suitably advances state organisations in other parts of Europe) meant that some projects could no longer be undertaken or financed. The simple fact that no one power controls a vast region means that no single government is planning a network of main roads, for instance. (And more modern communication and international co-operation were not yet feasible-- alhough in the later Middle Ages, there was far more of that than is sometimes assumed.) Architecturally, the increasing influence of non-Roman peoples who had never lived inside the Empire meant that there was increasingly less of a sense of "indebtedness" to the Roman way, and styles that owed little to Rome could arise. The same goes for art styles (and there, in particular, we see that Northern Europeans incorporated their own tastes and symbols very early on-- the Franks may have eventually claimed the Roman imperial title, but they weren't interested in looking like the old Romans.)
Then there is knowledge. In the posty-Roman world, this was mostly preserved by Christian authorities, for Christian purposes. The way information was presented served the interests of the Church, not the interests of the dead-and-gone Empire. Turning northern Europeans into good Christians was hard enough, and in many cases, the Church itself adapted a bit to be more acceptble to its new adherents. (Adapted, that is, in a way that automatically made the Church less Roman; less like the church it had been in the days of the Empire.) So why waste resources on an attempt to make Northen Europeans into good Romans, besides just good Christians, when the Church itself was actually becoming less (classically) Roman?
There are other factors besides these, but I've attempted to illustrate some mechanisms that were at work in this whole process. That way, we can begin to speculate on what would have to change, in order to make the Middle Ages "more Roman".
Firsts order of business: limit your ambitions to Southern Europe. Making Northern Europe more Roman isn't impossible, but the best way to achieve that is to keep "Romanity" stronger in the South, and hope that this will influence the tastes and sensibilities of the ATL North later on. What we want is less of a break with classical Rome, a more integrated state structure covering a greater mass of land, and a strong tie between that state and the Roman Church (so that the Church has a vested interest in being and staying truly "Roman"). So we want to avoid the Gothic Wars, is what I'm saying. Those are historically seen as the true beginning of the Middle Ages (and, one may say, of the much shorter period we might call the "Dark Ages"). Italy was devastated, what remained of Rome's legacy was no longer anybody's prime concern, and all-in-all there was a big break with the past. Much bigger than anything Odoacer ever did.
So prevent that. The barbarians who had invaded the Empire (who had actually been invited in, for the most part) were very interested in being as Roman as possible, early on. hat made them "legitimate". If we can prevent the whole mess-up, and ideally prevent Justinian from ever attempting re-conquest, we can aim for an ATL where this norm remains set. Where the post-Roman kings are actively interested in maintaining continuity. In an ideal scenario, we see a post-Roman, Germanic (but Romanised!) King achieve ever-greater dominance over what used to be the Western Roman Empire. This King should be formally a vassal of the Eastern Roman Emperor, and maintain cordial relations with Constantinople. This won't restore the WRE, and one may forget about taking Gaul back from the Franks, but Italy, Iberia and North(-West) Africa should be achievable. If some sort of "Kingdom of the Romans" can grow to cover that area, with the monarch theoretically ruling on behalf of the distant Emperor (which will just be a polite fiction)... then you're looking at a much more Roman ATL Southern Europe. This kingdom will automatically be the most organised and advanced state in Western Europe, which can be translated into cultural dominance. The Church, seated in Rome, will be much more interested in maintaining its Roman identity and its continuity with the imperial tradition.
We see here an ATL wherein Rome has less of a true "fall". The Empire decays and shrivels up, but instead of violent war and destructive anarchy, a new edifice takes its place. Less imposing, less powerful, but far more able to breathe some new life into the Roman tradition than anything that OTL produced. Southern Europeans can indeed wear togas and sandals in this ATL. Their architecture can deliberately be made to look like the classical Roman styles. Their symbols will be those of the Empire. All because of the prestige associated with them, and because they can be much more easily maintained/re-introduced in this ATL (with its greater continuity).
The effects on Northern Europe remain to be seen, of course. It could be that the Kingdom of the Romans is such a great centre of power that all of Europe will be greatly influenced by it in the end. It may also be that this more Roman state, with its more Roman church, will inspire greater anymosity in the North, and will be unable to spread its cultural dominance there. This can be a world where Southern Europe is far more Roman than in OTL, whereas Northern Europe is less Roman(-influenced). In a way, that in itself may count as a continuation of the way things were in Antiquity...
