Adem, because of the points I've outlined in my post. Doing a PhD is a specialised, demanding process. Registering for one on the basis of overall uni reputation is unwise. Given the high drop-out rate the most important thing is to assess the smaller things that will help see you through the doctoral process. It's not like choosing an undergrad course on a UCAS form, many other things need to be considered before the uni as a whole. Things like(in no particular order):
Funding, departmental profile, supervisory staff, existing PhD student progress and success rates, collaborative opportunities, personal development and postgrad training available, student support services, location...
Given the personal as well as academic effort needed to do a PhD, it's more important to look for these things first. Just because a uni flies in the league tables, doesn't mean it's the best one for the doctoral project one might wish to undertake.