To build on what SpiritCrusher said, it's mainly because larger and more complex maps are slowly being considered normal due to the rate that they're being released. Several years ago in the OLDC (before 2.0 was released), maps like Chemical Facility from CoatRack would have received near-perfect ratings because of
big it is. But today, because we're becoming used to such complex maps, the ratings were very polarized; it received mostly positive ratings, but it also got its fair share of negative votes and the overall consensus was that it was simply above average.
On the other side of the spectrum, Tree Ring Zone was released in 2005 and received perfect tens at the time. It was considered good enough that it was included with the vanilla game. If the map had been released today (with its object placement updated accordingly), I can guarantee that it would not receive the same scores. It would likely at best be considered an average level, maybe even below average. To use an older example, look at Nightmare Ring Zone by Mystic, released all the way back in 2003:
It looks terrible doesn't it? If this map was released today, I'm sure it would be universally panned by the community. But when it was released, it not only received a 7.75 in votes, it won the contest. Back then, this was considered good. Great even. But as more complex levels are being released and certain elements that were once rare become more commonplace, the standards rise until what was once considered great is simply average, and then in time, mediocre.
As for why people are pessimistic about Sonic in general, that's a whole other can of worms and is probably best saved for another thread.