Nobles Adopting Children

In the middle ages, the bloodline was seen as very important. but in the Roman Empire that came before, it was not only Emperors who would have adopted sons as heirs, but it can be seen that owners of large estates did that too.

So why didn't it continue into the 800s? Boarders might change as Rome becomes no more, but what made the noble class to decide to be picky on bloodlines? Why didn't Europe have something like Mukoyōshi like the Japanese did (or the Romans before them)?

Let's say the divine right of kings existed as a concept. OK, but Roman Emperors ruled through the pagan days and Christianity and in the eyes of loyalsits ruled with the gods'/the Lord's approval and while it was united as an entity people didn't question adoption. I'm not saying everyone has to resort to that if they find themselves without a biological offspring (I'm sure some brother, sister, uncle, or cousin can do as a heir) but why is the adoption option gone?
 
Rome, even deep into the days of the Empire, was basically a crowned republic. So what really mattered was the position, not the blood. The early kingdoms used some of the same rhetoric that Rome did, about being chosen by God, but new developments like the Salic law made that more about a familial lineage rather than hypothetically serving the public. Thus came the idea that the state is the private property of the ruler.
 
Also, in the absence of solid institutions, I'd say the bloodline might be a good way to ensure stability.

With adoption, everybody is potentially legitimate and you can always resort to "he secretly adopted me" or "he designated me as his heir on his deathbed, shame you weren't here!".
With a bloodline, there's way less doubt about who has to be in charge
 
With adoption, everybody is potentially legitimate and you can always resort to "he secretly adopted me"

No, never happed to the Romans. Adoption by rulers was always a public event. Same with Japanese Mukoyōshi. When a biological child is lacking in either talent or existence, the adoption of the in-law was a very public event to make sure everyone knew. If adoption was a thing, I'm pretty sure no one falls a secret adoption claim. The rest of the issues on stability I suppose might apply, but Imperial Rome was pretty stable with a short break in The Year of the Four Emperors (I blame Nero) until the Crisis of the Third Century where things got pretty bad and even after the crisis was over, the Roman Empire and later the Western Roman Empire was not exactly stable, even though they might periodically regain their Trajan boarders for a decade or so.
 
In the middle ages, the bloodline was seen as very important. but in the Roman Empire that came before, it was not only Emperors who would have adopted sons as heirs, but it can be seen that owners of large estates did that too.

So why didn't it continue into the 800s? Boarders might change as Rome becomes no more, but what made the noble class to decide to be picky on bloodlines? Why didn't Europe have something like Mukoyōshi like the Japanese did (or the Romans before them)?

Let's say the divine right of kings existed as a concept. OK, but Roman Emperors ruled through the pagan days and Christianity and in the eyes of loyalsits ruled with the gods'/the Lord's approval and while it was united as an entity people didn't question adoption. I'm not saying everyone has to resort to that if they find themselves without a biological offspring (I'm sure some brother, sister, uncle, or cousin can do as a heir) but why is the adoption option gone?

You are only partially right, in the meaning that bloodline was very important for the Romans too.

As far as sources can let us guess, childless Romans, either noble or non noble, most often adopted relatives, be they nephews, grandnephews, grandsons by one of their daughters, or cousins, or the husband of one of their daughters.
 
Plausible, you need to attain the right conditions to keep the institution alive but there's no reason that it can't stay around. Just avoid pesky things like Salic Law.
 
My point was that the bloodline was considered important, but they didn't cut themselves off from other options.

Yes. But except a few exceptions, they always chose relatives if they had one to pick.

They even invented testamentary adoption and, best of all, posthumous testamentary adoption. The most famous case of posthumous testamentary adoption was Marcus Junius Brutus, Caesar’s murderer, whose adoption by his childless deceased uncle Quintus Servilius Caepio (younger brother of Servilia and last of the male patrician Servilii Caepiones) was organized by Servilia to secure official continuation of her paternal patrician family line.
 
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