Over-saturation of Trek might have been part of the problem. But another part of the problem was that they had stopped taking chances with the property.
For the most part, episodes of Voyager and Enteprise could have been episodes of TNG with a little re-writing. both of these seires had initial premises that could have lead to new and interesting story-telling, and they seem to have mostly wasted these opportunities. I will, however, give Enterprise credit for trying real hard in it's last season to turn this trend around.