I don't get why people don't consider the future episodes canon

There's a simple rule in simpsons if its halloween or christmas its not canon. Obviously having watched simpsons loads if you add up all the pieces it all works and even if you consider episodes from 20 years ago we have an actual timeline of events that make sense.

Probably because the characters never age but the years still pass. Lisa was supposed to get married in 2010 or whatever but it's gone past 2010 in the show now and she's still 8 years old.