IMHO:
After soo many successful series, they need to take a break, start fresh and approach the next series from a new different perspective.
Enterprise clearly demonstrated that they had run out of ideas, and were drawing off of old material constantly. [No matter how well they attemped to dress it up and or disguise it, it was old concepts, old episodes, redone. For a true trekkie, it was all too evident from day one.
The original Star trek's demise was that they wanted to take it to the next level, because the technology and special FX had out grown the old format and was ready for a change. [Well written]
TNG's demise was that they wanted to and was axious to move that format to the big screen. [excellent writers]
Voyager's downfall was that they traveled back in time every other third episode. [No creative writers]
DS9 was taken as far as they could take it. After the dominion war concluded, they had no where to go with it. [Good writers for this as well]
Enterprise [They totally loss the flow ..... BIG TIME]
[Not well written at all ... No forsight]
Hopefully the best is yet to come.