So, uh, basically, there are 2 main reasons: The nuke not registering everything (mostly fixed, but not quite yet) and it's overall a better practice to stop using Incremental Counters.
Another reason i think of the randomized IDs are because they're trying to inflate the user count.No.
So, lets get into our first reason: The nuke.
So the nuke removes a user permanently from the SQL servers. However, there are 2 problems: It's currently incomplete and it can't remove cookies (no, not sessions. Cookies). The nuke cannot and never will remove cookies, as that's currently stored on your PC. Meaning that unless we force the site to be a Electron app (and I know how many of you like those /sarcasm), it'll still linger on your PC. Now, I'll tell you the story of a board.
So, I was on a board, and they still had incremental counters. Now, I registered using a proxy, and that derp (not mentioning him because of drama) nuked me (for no reason, sure, but that's besides the point). Now, during the time I got banned, another user registered under the name Luigi_Fan. I changed my IP, so that I wont be banned and BEHOLD! I was able to access that account. Why? I never deleted my cookies, and there was no IP_Ban under the new IP. The sessions messed up, so I was enabled access. For that, the bug relies in the nuke, but there are a bunch of other things that aren't deleted. One of those being Permissions. So I decided to remove all complications, and just randomize it.
Although this does restrict us to strictly 999999999 and not 1000000008 ID's possible, we'll most likely never reach that number anyways.
Now for the second reason: Better Practice.
Watch this video until 3:20.
BTW, uh, just saying, not trying to put blame on him, but rather, trying to give proper credits: This was all made possible by Not Jon.