Why didn't they just disable MCAS?
1. Lion Air pilots (all of them) didn't know anything about the MCAS. Boeing is under fire for not telling anyone. Call it an "undocumented feature".
2. Because Lion Air pilots didn't know, they also didn't what it "feels" like with the MCAS takes over.
3. The flight crew from the previous flight "blindly" disabled MCAS. They didn't do this on purpose. They misdiagnosed the issue and turned off the trim which resulted in gaining control of the a/c.