Because Disney decided to pander to the nose-picking-Playstation-zombie demographic who thought Horizons was "boring".
Don't get me wrong...I actually do enjoy Mission Space as a thrill ride. It works. (lol: MS Works)
BUT...Epcot Center, by virtue of its mission statement to "entertain, then inform and inspire all who come here", was never meant to shift solely to thrill rides. (this is also one of the reasons I completely fell in love with Test Track 1.0 and absolutely cannot stand Test Track 2.0)
MS could have been something truly spectacular if they did like Wonders of Life and made the thrill ride just one part of a much larger experience. But they didn't. They went full in with "EXTREME THRILLS", and once it made people sick/dead, they partially shut down half of it.
And they also put in those ludicrously long hallways.
So very sterile. So very bland.