Because people want to stay stuck in the 80's forever. Epcot's "heyday" was probably 1989 - Wonders of Life had just opened, Norway pavilion opened, The Seas opened in '86, Kitchen Kabaret was around, they hadn't given up on Horizons yet, I assume this is the time period when Peter Quill visited, etc.
1989 was only 7 years into the park's life however. The park VERY QUICKLY started to go down hill in the 90's and by the end of the decade, the EPCOT Center concept was pretty much gone and the park was in rough shape. To this day it still is in rough shape and has had very minimal changes to the park since the Millennium Celebration. Mission Space opened in 2003 (was already announced in 1999), Test Track has simply been given an overlay, Soarin' replaced KK/FR, Nemo took over the Seas, Imagination got worse, SSE had a narration change and unfinished finale, Cablleros took over El Rio and Frozen took over Maelstrom.
Notice all of that was either updates or replacements. Nothing has actually been ADDED to the park in that time. Actually, the park has lost attraction capacity as Wonders of Life is sitting unused to guests. Innoventions has closed as well. The Magic Eye Theater is all but closed right now too with them showing Pixar Shorts in there. Ratatouille will be the first park expansion since the Summer of 1989. It's also a thematically appropriate one.
IPs are not a bad thing, it's just about placement. GotG is an example of bad placement as it feels like it would've fit better in Tomorrowland and Tron fit better in Future World. It does suck that the showbuilding will be so large and seen from so much of the park. It's incredibly poor design imo.