The original Monkees may have been a manufactured band, but they were presented as long-time friends who had been playing together for years. And the actors had the skill to make the chemistry work.
The New Monkees let you know right up front they were four rando actors thrown together. And they had neither the writing nor inclination to act like they were a group. It was four assholes, each out to upstage the others.
THE SECOND BIGGEST MISTAKE:
The original Monkees were unconventional. It was still the 1960s and the younger generation was challenging the status quo. Not everyone "got" them.
The New Monkees were a product created for the broadest possible appeal.