Once the second crash happened there was extra oversight and other issues that were signed off on during the design phase were once again put in question. Its no longer just MCAS being modified but what happens when the two flight computers disagree.

So to answer your question there are now multiple issues at play and they are effectively redesigning the flight computer system as a whole. This is not a quick fix.

Because rushing a fix into production and then having another airline crash would be far more severe than the current situation.

So it makes sense to step back, and fix things the right way before you reintroduce the system back into the market.