Their stock price has been down considerably since May, around the time of Wrestlemania.

That may be more related to the fact that their stocks always shoot way up during Mania season, and come back down after.

I think having Brock beat the Undertaker rubbed a lot of fans the wrong way.

I think a lot of fans were upset at first, but shock played a bigger factor with that than actual disapproval I think. For a young guy to have broken the streak may have put a lot of pressure on them. What happens if the dude beats the streak and then gets an injury at the next PPV that sidelines him long enough to kill his momentum? What if his career for whatever other reason doesn't take off? Obviously you would expect if he beats Taker his career will, but if for any number of random reasons it doesn't, you just dropped your most impressive streak to a nobody. It had to be someone who was relatively established, and who still had potential to stick around for a while. I do agree that I hate that he is a part timer, but at the same time, after beating the Undertaker and with his resume prior to the big win, Lesnar is such a HUGE PPV draw now that when he does defend the title, it's all the more special. The title is more prestigious now than it has been in quite a while, and it being on Lesnar after beating Taker has a lot to do with that I think.

If Undertaker should have lost his streak it should have been to Randy Orton years ago, which the Undertaker actually wanted to happen.

I'd have loved to see Orton, Punk or Edge do it. Of course, given the three from WWE's perspective now, obvious choice is obvious... And still only kind of considering Orton's injury issues as well. And for what it's worth, Taker wanted Lesnar to end the streak. Source