I agree why rip up your own meal ticket, which would be my first thought but how could this show miss with those writers it just doesn't make any sense!
Burn-out and overkill. At least for me...
Star Trek: Enterprise
represented the 22nd, 23rd, 24th and 25th seasons of Modern Trek and, honestly, it all started to blur together. There's only so many times you can do the holodeck creates an inconvenience, the Klingons are mad, the Klingons mumbling about honor, the Vulcans don't have emotions, the captain struggles with whether to help primitives, the shields are failing and so on.
As a series, there just wasn't enough that simply felt fresh. One out of about every ten episodes they made would wow you with something really great, but that's a really poor batting average for a TV series.