Because WWE seems to only know how to book faces two ways, either as the sympathetic underdog or supermen.
Pretty much this right here... The OP also nailed a good bit of it his post.
WWE / WWF have traditionally booked their baby face champs as supermen.
Bruno, Hogan, Backlund, Cena, etc,..
All have standard "hero" booking..
1. A new threat to the hero / champ emerges..
2. The new threat is built up as real danger to the hero / champ
3. After a struggle the hero / champ finally beats this great threat...
4. Pick someone on the roster to be the next threat and goto 1 above - repeat for several years...
That worked fine in Bruno's days, even Backlund's and Hogan's and some of Cena's..
Honestly, I think part of what happened was they over-did the amount of wrestling on TV. Back in the day what i described above might take months to play out before the cycle would start again..
Now with 7 hours of wrestling on TV every week plus PPVs the cycle has really sped up and the same villains / threats are being recycled over and over again week after week..
It gets boring...
Think of it like this...
One new Batman comic every month and you are waiting to see what happens next. This is how it used to be with wrestling too..
Change it to 2 or 3 new Batman comics a week all cycling through the same villains and you would get bored pretty quickly because you would be saturated and already know how the story would go before you even read the comic. This is how it with WWE now....