We obviously will never know the details but we do know that Bethesda shopped around for a buyer.
Sony likely were either willing to offer less than Microsoft was willing to offer or weren't interested in Bethesda, period. So basically, Sony didn't feel that Bethesda would have been worth 7+ billion USD (if it went into a bidding war between them and MS).
Just like Bungie shopped around for a buyer, but MS weren't interested in them either at the price Sony were willing to spend or weren't interested period. The upcoming ABK deal, obviously altered Microsoft's valuation of Bungie. If the ABK deal weren't on the horizon, I could certainly see MS being interested enough to outbid Sony to acquire Bungie.
All of these deals are conducted out of the public eye, but they are often far more nuanced than people speculate. Did X deal get turned down because a party wasn't interested? Because they were interested but the price was too high? They were interested but another more interesting opportunity presented itself? Etc.
Microsoft likely overpaid for ABK. Why would they do that? To avoid a lengthy drawn out bid war in order to get the deal done as quickly as possible. Plonking down a large almost unreasonable sum of money would serve to scare off other interested parties leaving ABK with only one suitor and thus cut down the process by multiple months. They knew that they would have to face regulatory approval and expected that to take the better part of a year, but they didn't want to have additional months of negotiations and bidding on top of that. IE - try to get the deal closed and approved in roughly 1 year instead of it taking roughly 1.5-2 years.
Regards,
SB