Why do tonnes of unfinished series suck? because they pressure you. When you think about all those series you have started but haven't finished you feel pressured to read them creating a dislike for them because of said pressure which in turn makes the "I don't want to read you right now!" feeling worse and you get further and further away from reading those last few books.
Then there is the "I shouldn't be reading this" idea. when you really want to start a new series or standalone even, but you are plagued with the feeling of all those other unfinished series and story's that you have yet to read which then makes you feel bad for picking up a book that doesn't go towards the goal of finishing those series.
I'm slowly getting though books here and there that go towards my list of unfinished series but the climb is still slow because of the fact im a mood reader and the things above affect me in a big way. I hope by the end of the year to get a few more series done or caught up on so there is a little less pressure going into the new year. One of my resolutions for that is no more than 3 unfinished series at a time compared to my 20 something at the moment.