Maybe because there isn't much practical point to a more powerful .44 than the mag. As such, the only people who would be interested in one are just ones who want the biggest handgun possible. And well, the .44 mag isn't the biggest by far anymore so all the power junkie's interests have just moved on to other bigger calibers to play with.
If you're gonna create a ridiculously powerful revolver cartridge, no sense starting with a .44. might as well start with what's already the biggest like a 480 or 500
Basically, no practical market for a more powerfull .44 magnum, and no "just for fun" market for a more powerful .44 magnum.