I'm surprised myself too, because Wrestlemania sells hugely off brand name alone, hence why it can get a million buys or close to it every year regardless of what the card is, while every other PPV (except for the Rumble, which also largely sells itself on brand name) sits around the 200,000 mark or lower. Then there's the fact that it (and nothing else) can sell out stadiums or come close to it just like that. This year's Wrestlemania even sold out the fastest in years.
When you add in that most fans usually don't care about predictability when it comes to a big event like this and the card had two matches which drew huge before (not to mention that Rock and Brock's cross over appeal alone should have drawn in interest) then 600,000-700,000 is indeed surprising. I was hoping it would go below a million since given who and what was on the card, that was be considered bombing. But going the lowest a Wrestlemania has gone since a decade is surprising.
I guess the decision to put the title on The Rock didn't pay off as well as they hoped in the long run. Don't forget the Rumble also did a disappointing buyrate in WWE's eyes and put Vince in a sour mood, as they hoped it would go at least above the 700,000 mark.
The funny thing is, if the final number do indeed show the buyrate being this low, it's ten years in the making. It would be the lowest buyrate since WM X-9, which also was a show loaded with talent (Hogan, Vince, Austin, Rock, Michaels, Triple H, Angle, Lesnar, etc.) just like this year but it still bombed.