The million-dollar question here, IMO, is how likely or unlikely it is for there to be a number of continent-spanning religions wiping out most of the interesting religious diversity. On the face of it we might infer that it's more likely then not, just from the fact that their have been at least three OTL, with Hinduism and the assorted East Asian faiths being the only non-big three faiths holding a majority in a meaningful swath of territory.
If we were to try and make an argument against that though... we might observe that Buddhism is the only one of three that really spread itself on it's own merits. Christianity and Islam wouldn't have been half as successful if they hadn't been hitched to Rome and the Caliphate respectively. They then proceeded to win additional converts mostly due to having dominated such a large amount of territory that converting was politically and/or commercially beneficial for the converts. As for Buddhism, it fell to pieces in India under the weight of Hinduism's resurgence, to say nothing of the difficulties it faced due to persecution in China.
So maybe Christianity and Islam's near-duopoly outside of the South-East quarter of Asia could be considered a historical fluke? After all their were plenty of long lived empires that didn't adopt and coercively impose a single monotheistic faith. And there are plenty of empires who's founders weren't religious prophets. Rome and the Caliphate stand out as the exceptions, not the rule.
It's also worth noting that they were both unusually large. No other empire has come close to uniting as much of the Euro-Mediterranean region as Rome did, nor has any empire other then the early Caliphates maintained longstanding control over both the Eastern Mediterranean and the Persian plateau. So, if we assume their size is a fluke, maybe we could presume that their ATL analogues are likely to dominate far less territory early on. And given that their later expansion mostly relied on either a)conquerors arising from the established conquests, or b)pagan conquerors deciding it was too much bother ruling them without converting to their faith or c)coercive trade monopolies... well, the smaller the base that's initially established the less the distance those three factors are likely to carry them.
TLDR: It might be that the territorial scope of Islam and Christianity is a fluke, given that most empires weren't founded by or converted to a new and absolutist faith, and given that most empires weren't able to maintain long term control of such wide expanses of territory as the Roman Empire and early Caliphates did. So, in our hypothetical multiverse, it might be that our planet is abnormally undiverse when it comes to religion.