Why is the WoW expansion Cataclysm only 5 levels

The official "story" from blizzard.
Why is the level cap increasing by five levels (to 85) instead of 10 as in previous expansions?

We decided to focus on providing more end-game and maximum-level content for players. We learned a lot from Wrath of the Lich King, and want to ensure players have more content to experience right away when they reach the new cap of level 85. We’re also putting a lot of time and effort into reworking the level 1 to level 60 content. We’ve learned a lot designing the previous expansions and felt we could apply those lessons to improve the leveling-up experience. Creating alts has always been a popular pastime for players, and we wanted to ensure Kalimdor and the Eastern Kingdoms remain a vital part of the game world.

Here's my take on it:
They want, in part at least,  to ensure that what happened to Burning Crusade after Wrath was released doesn't happen to WotLK after Cataclysm is released. That being the ability for level 80 players to go in and trash any level 70 end game content without even breaking a sweat. Although it will still be possible to solo some of the Wrath instances at 85, those extra five levels would have made level 80-81 mobs a grey con and the instance a cakewalk.