Boeing decided it was best not to tell pilots about MCAS as they didn't want to overload them with information. It was assumed the standard procedure for dealing with runaway trim would be sufficient, except in this case it didn't present as runaway trim because it's limited to 2.5 seconds of travel at a time.
So the pilots were attempting to diagnose a problem without knowing all the potential causes, it's therefore likely they misdiagnosed the issue and were applying the wrong solution. n.b. there is another automatic trim system (STS), for fine tuning the pitch as speed increases, that they would have known about and may have thought was the issue.