Because apart from MRCA we also have to negate large fleet of MKI's that will be somewhere around 270 mark and also FC-20's are inducted in limited numbers ,just 2 squadrons as of now,there are rumors that the order may go upto 150 but still you can't bank on rumors until and unless there is some solid background .
The twin engine of J-10 which has been rumored around as J-10C is still nowhere near reality ,we even don't know that will it ever materialize or its just a paper concept .even if there is development going on of J-10C still we can't go for it as it will take a lot of time for PAF to induct them and time is essence in this case
Secondly Twin engine doesnot always mean that it is better ,when you can accomplish your purpose with single engine then whats the need to burden yourself.
And yes according to AFDP PAF is refrained from inducting any twin engine aircraft until 2019.Yes we have operated twin engine aircrafts in past but that was a different case