It's a matter of common sense to answer this question. Almost like mathematics 2+2=4 !
Army wants military-supremacy in Pakistan in near & far future.
Who does suit them most on the PM's seat?
Nawaz Sharif, a certified dishonest, corrupt and weak leader with less public support.
Imran Khan, an honest, strong and popular leader with much larger public support.
Both Military & Civil establishment want Nawaz Sharif/Zardari in power and Imran Khan as a pressure group only.