Because it would be cool and special, and Blaze is still good live. And I doubt we will see a reunion with former members live (even for one show), so there's that too. They are good friends with him.
Edit: this won't do Blaze any favors, especially next to Bruce, but that doesn't matter anymore (with the solo career he has now). A one-off event celebrating Maiden's history would be great. Bruce is the best for the band, but every former member of it is an important part of the journey. Blaze always will be associated with Maiden. I think he's better accepted among the fans now.
This.