Read everything here: Microsoft changes default Flash behavior in Windows 8 and RT
Microsoft’s official announcements say the change is based on an ecosystem that has gotten better at developing Flash content. But I suspect the real reason is more pragmatic. This behavior was confusing to users and frustrating to developers. For Windows RT in particular, it had a devastating effect on some sites, which simply wouldn’t work, and the fact that you can’t install an alternative browser on RT eliminates that workaround. And at this point in its life, the last thing Windows RT needs is another reason for potential buyers to reject it.