Thor's hammer served him as such due to one of the most powerful enchanctments possible- that being the will of his father, Odin. Things have changed over the years, but when Stan Lee was writing the title, he essentially saw Odin as being Marve'ls stand-in for God (Jack Kirby designed him resembling Michelangelo's God).
Anyway.. Doom wouldn't have the kind of power to circumvent that.
And logically, since all of the Asgardians are supposed to be recently deceased, the enchanment should really be gone. Mjolnir might well still be intact, but it would likely merely be a hunk of uru metal, with no special powers. This is evidenced by the fact that Odin had more than once removed Thor's godling powers, rendering Mjolnir no more than a classy paper weight.
The one other LONG SHOT, I can see in the Doom plot is that this is Marvel's round-about way of bringing Thor back. That perhaps Thor's spirit exists within the hammer, so it isn't functioning for Doom as it would for him, but working through Thor somehow?