Why do conferences keep deferring the deadlines?
There are basically two approaches for conference deadlines.  In my experience, top conferences have a "hard" deadline and do not slip it, and the rest frequently slip it once.  Usually the slippage is actually planned, and it's announced pretty far in advance: for instance, a conference might say 2 weeks before its announced deadline that there are actually 3 weeks to go.  Sometimes it's a panic because of low submissions, which is probably a bad sign.

Notifications are another story.  It's considered poor form to miss the announced deadline, though if the change takes place before submission (delaying both submissions and results) that seems ok: people can choose not to submit.  Simply missing and redefining the deadline isn't playing by the rules.

I wouldn't say that the changes to the deadlines are a reason to avoid a conference.  Many factors go into the decision about which conference you submit to.