It says here:
Twitter suspends accounts which violate the Twitter Rules
This issue recently came up on the main Meta as well. I'm quoting an answer by @double-beep:
It seems that Twitter suddenly and randomly started blocking those accounts because they were bots posting automated tweets and that resulted in SE not creating new Twitter accounts for new sites since 2015. It's still not known why Twitter has deleted only some of the accounts.
Tracing through the links in @Glorfiendel's answer, I found this answer which gives a little more info:
Not all (Stack Exchange) sites have Twitter accounts. We stopped creating them for all public beta sites by default because a) Twitter sees very little engagement for us; b) Twitter-the-company started blocking new accounts for us (probably because it thinks we're bots?) and fighting that got too annoying and time-consuming.