There is no Diffie-Hellman but Elliptic Curve Diffie-Hellman

When I first learned about Diffie-Hellman and especially elliptic curve Diffie-Hellman, I had one rather obvious question: Why elliptic curves? Why use this strange group that seems rather arbitrary, with its third intersection of a line and then reflected? Why not use, say, the Monster Group? Surely a monster is better equipped to guard your [ ]