Drosselmeyer fully intended to make good on
his bribe to the Sorting Hat, and so:
My good sir,
I hear that you have quite a talent for building, and I find myself in need of a puppet theater. I hope that you might offer your services in constructing one for me. In either case, I shall wait for your reply.
Sincerely yours,
Herr Drosselmeyer
After all, he was a bit lazy at heart and liked to pawn whatever work he could off on others. The theater itself seemed safe enough to entrust to someone else's hands.