The most obvious reason would be that you don't use it, don't enjoy it and really are getting nothing out of it. If you hold a Twitter account that you don't use, most of the posts you get really are pointless and you may as well not bother.
However, some people close their accounts after tweeting regularly, as in this 2008 case. People can find that they're almost getting addicted to tweeting and feel they have to make every tiny detail of their lives public; and as well as the time you waste doing this, you also waste lots of time following other people who are also just reporting trivial details of their lives.
A third reason for some people - often, but not only, celebrities - is that they get some unpleasant reactions. If you attract hostile or even threatening tweets, you may well feel that you don't want to be involved any more.