ISLU also had a very good team and a very good chance to qualify, but they messed it up with their own lackluster form. One only needs to consider Imad's batting averages in domestic competitions to realize that he is a far better batsman than Shadab.
Imad Wasim:
FC Average: 40.7 SR: 49.8
List A Average: 37.5 SR: 100.7, his ODI SR is 110.29
T20 Average: 12.8 Average is actually 19.52 with a SR of 126.55
Shadab Khan:
FC Average: 27.1 SR: 67.61
List A Average: 26.2 SR: 78.76
T20 Average: 15.9 SR: 133 and average is about 17.88
As far as the bowling is concerned:
Imad Wasim:
FC: 77 games, 141 wickets, 31.14 average
List A: 116 games, 117 wickets, 36.57 average, ER of 4.68
T20: 200 games, 172 wickets, 25.39 average, ER of
6.47
Shadab Khan:
FC: 17 games, 68 wickets, 25.77 average
List A: 56 games, 80 wickets, 29.16 average, ER of 5.21
T20: 139 games, 164 wickets, 21.82 average, ER of 7.1
On paper, Imad is a better batsman whereas Shadab is a better bowler, though I rate Imad much higher for two reasons:
His batting is good enough to be on the ODI team alone and his economical bowling style in the powerplay is necessary for the team.
Secondly, he offers a lot more batting with a very little compromise on bowling, because Shadab's batting isn't too great to call him an all-rounder, and the team could replace him with Usman Qadir without much harm.
Both are good players, but moving to your question about why Imad doesn't play tests, that's a mystery to me as well. I read some interview/article a few years ago, and I don't know how reliable it was, but it said that he didn't play tests because his body couldn't handle it.
[MENTION=149054]shamaan[/MENTION]