I think they caved to the stereotypical requirement for expansions to be harder than the previous game, as well as the demands of the vocal videogamer minority for "more challenging content". Also, power creep, except only on the mobs side.

Of course, given what happened to Orr after the game originally came out, we can predict what's going to happen eventually here: HoT will come out. The casuals will play it and get frustrated, finally motivate some parts of this largely silent majority to finally start complaining about it on forums and such. And then Anet will nerf the hard things or add more options to make it easier.