The problems could be related with the fact that pangolins have an unique and strange immune system. Reduced in comparison with most other mammals (but I'm speculating...)
Immune systems are fairly differentiated across species, even for common mechanisms. Source: I worked from 2009 to 2011 in a project aiming at reconstructing part of these mechanisms in certain immune cells, and the differences between human and mice, where some models were made, were significant.
This virus definitely looks as if it had ample time to adapt to human host. Just many early cases were probably undetected or misattributed.