Slight reframing of the issue starting with some assumed true statements, mostly applies to both accidents although the lion air prelim report does not show control column positions.
1: The pilots were initially aware that the plane was out of trim through no action on their part.
2: They counteracted some but not all of the MCAS inputs.
3: The last moments of both flights saw brief manual trim inputs, woefully insufficient.
The question is why did both crews only partially retrim and then loose it at the end?
Possible answers, not mutually exclusive of course.
A: They were confused by all the warnings and just lost the plot.
B: They had no training with grossly out of trim conditions and were used to short inputs.
C: Something confused them after the initial re-trim to believe the airplane was close being in trim, despite the obvious control column in lap.
Could they have believed that some other factor was causing this?
D: Some biomechanical or other issue prevented them from actuating the switches.
As others have stated 'basic airmanship' should have worked but clearly something was missing.
Note, There were also a large number of other factor that could have prevented these accidents, especially ET, this is just about why the crew did what they did.