This is a topic for fans to rightfully be emotional for, however I'll try my best to list out the most objective, underlying issues for Pakistani cricket since the Champions Trophy triumph.
The following numbered points should are problems specifically prevalent throughout the last two year patch. I'll mention these issues first, except note these are problems that have hurt Pakistani cricket for far longer than just 2 years: Refusal to adopt fitness and fielding standards (has fielding at times improved? Yes. But we are still treating fielding as an accessory to batting and bowling, rather than an essential 3rd facet of the game), and failure to adapt to modern-game (whether in scoring rate, straying away from bowling at the wicket or completely straying away from yorkers, etc). Also, we have never been a good chasing team and that has sewed deep into our culture.
2017-2019
1. Over-reliance on our top-three
2. Failure to take wickets in the powerplay
3. Failure to take wickets in the middle phase, particularly Hasan Ali who has maintained his poor economy but tarnished his frequency in wicket-taking. Excluding performances against AFG, HK, and a performance against Sri Lanka in Oct'17 which earned him the MotS award due to his 14 wickets in 5 matches -- since the start of 2018, Hasan Ali is averaging nearly 65 with the ball at 6.5 per over. (This guy was half of the reason we made it to that CT17 Final)
4. Assuming Amir is a primary strike bowler off of one 3-wicket performance against India
5. Inconsistency from PCB and selection
6. Continued persistence in weak middle order (and continued exclusion of Haris Sohail lol)
7. Having a captain who often fails at holding his weight as an individual batsman and fielder in the playing XI
8. Choosing to bowl first when our team continues to fail at chasing against good teams
9. Over-reliance on predominantly bowling all-rounders to anchor an innings