When you're short as a pacer, your margin for error is far less because the lift from the pitch is far more easy to deal with compared to someone who is a lot taller. Lets not forget Hasan Ali isn't just below average height (around 6ft - 6"1), but he falls well short of it, standing at just 5"7, so for him his margin of error is even less, compared to someone who's 5"9 - 5"11.
During Hasan Ali's purple patch, he was bang on the money with his line and length and as a result had confidence in using his variations/skills such as the slower balls, reverse swing, surprise short ball and etc.
Also his lack of height was also exposed in SA (earlier this year) on those harder wickets. Whenever he pitched it up, it bounced at nicely for the batsmen to drive and when he went short the batsmen were quick to smash him on the back foot because of the lack of lift and pace in those deliveries.
If you're not a 145 km/h bowler or possess supreme bowling skills, it's very difficult for someone short as Hasan Ali to sustain consistent out-performance.
