Monk; two-part episodes in general, and of course Star Trek

TV Arts

They just aired the second part of the two-part Monk finale.  I
thought the second part wasn't as good as the first, and the two parts
had different writers, though all the writers are regular writers for
the series.

Do you know why two-part episodes are sometimes not all written by the
same writer?  In Star Trek that may have happened though it didn't
happen with The Menagerie.

On the other hand, the episodes of a "24" season were often written by
different people.
[email protected]:

Only about 1 show's worth of new material, if that much, was done for those 
episodes.  The bulk, it seems, was from the orriginal pilot.