How do I tackle the interview question "why join here and not your former college"?
If they ask, be diplomatic. Slagging off your old institution doesn't send a good message. Something like:


  While I enjoyed my time there and feel they helped me to grow into an independent researcher, I'm now looking to gain experience of working in a different department in order to develop these skills further. 


might work, if true (or at least true enough for a job interview). 

Then expand on this by explaining why you chose to apply to this new institution, rather than elsewhere, by explaining what you see as their positives.