Why is this in the imperfect subjunctive?

Hola Anthony 

There is a general sequence of tenses in these cases, so if you have the conditional in the first clause (me gustaría que...) the subordinate clause most of the time uses the imperfect subjunctive. The use of the present subjunctive instead would have an extra nuance. Here is a kwiziq lesson that talks about that precisely. Have a look here. 

I hope it helps.

Saludos

Inma