They misjudged the pitch, it only got slower in the first T20I as the match progressed which they didn't seem prepared for.
I think the main reason they opted to field was the dew. Both Sarfaraz and Misbah had mentioned it before the series.
Judging the pitch is also part of planning - this is not UAE and PAK was playing at the heart of their cricket HQ!!!! Regarding dew, I believe it was 6:30pm start means by the time dew really starts to affect (around 9pm), a T20 game should be almost over. Against this inexperienced SRL lineup, what PAK needed was to bat out 20 overs and then make the chase tight - they would have crumbled under pressure.