Really? I think its the span of 4 years that caused people to lost interest, but DP had enough going on in it to keep me interested. Sure I almost zoned out, but when I saw the new titles and something interesting kept happening, it kept me engaged.
The problem is they stretch out sagas over 4 years, so people just move on by the middle. If you watch a saga all at once, its much better.
DP did indeed drag, but it probably is the fatigue of the entire series going on for 13+ years.