It's clear from all the faux outrage in this thread who actually played the game and who didn't.
For starters, and by far the most important reason, the online in this game is a ghost town. The community was never large to begin with, most have moved on, and it was pretty buggy to boot. We're talking like twenty or thirty people here, tops. Nobody would pay to keep servers up for an annualized game with a couple handfuls of people still using them.
Sports games by their very nature have short legs, and people who buy them tend to know that support will get dropped not too long after the next iteration. 20 months, as is the case here, is actually above average for this kind of thing. 2K tends to pull the plug in 12-18.