Because of the potential significance that is added to the characters when they are brought to new heights. This is mainly speculation right now, but given Disney's track record, this outcome is likely.
Yes, Wolverine is popular. Yes, we've seen him in movies since 2000, however, Disney will probably make him (and the rest of the X-Men) even more popular. This isn't going to be just another series of half decent X-Men movies that FOX butchered in so many ways, because it's Disney this time around, and people are expecting a new dawn and a new era for the X-Men, something we haven't seen before. This is what the speculation is all about, and is what's driving up the value right now.
It's not just Hulk 181 either. The major X keys are all going up in value, and if Disney's X-Men films are successful (and they probably will be) this increase will not only be sustainable, but will also become the new norm.