Microsoft own Mojang, they want to centralise the account system to one Microsoft account rather than having Microsoft account AND Mojang accounts.
Mojang accounts are not secure, as can be seen by the amount of stolen account issues out there, MS accounts have alot more security and 2FA.
The single account system will also make it easier to have cross play, if Mojang ever get round to getting the java and bedrock editions to talk properly themselves
Mojang accounts aren't secure because the people who actually worked on that account system must've been genuinely brain dead, for years you could know an unmigrated account's email as long as you knew the username (which was easy to find, just join a multiplayer server, done), that bug was patched 6 years ago, for years if you could find the answer to 1 security question you didn't need to answer the others if you knew how to code, this bug was patched a year ago, for years you could login to an account with unknown security questions as long as you knew the security questions of another account, this was patched a year ago, for years you could change the email of an account without security questions with a bit of code, this was patched half a year ago.
If whoever worked on the login system wasn't absolutely brain dead all of this wouldn't have happened, hell I think I myself could do a better job than those idiots and I barely know how to code. It would be incredibly simple to have a steam guard like system where it sends you a code by email or phone, but no our only choices are Microsoft which sucks on a privacy standpoint and mega sucks on a support standpoint, or Mojang, which sucks on a security and support standpoint (at least when I need to recover a mojang account, mojang actually helps even if I have to wait for the heat death of the universe to get a bloody response, microsoft just tells you to go away)
Also I don't care about crossplay as it's better with geyser than with official support which would mean having to deal with the bedrock moderators that are allowed to just ban you from the entirety of multiplayer just because they feel like it.