Because it was produce in conjunction with ADI who already had existing molds for the dog/ox alien. The drone was done by Stan Winston Studios. Alex Gillis and Tom Woodruff who own ADI worked for Stan on Aliens however ADI did not exist.
Rather than the dog they could have produced one from Alien Ressurection or either of the AVP films but I have the dog and I doubt that any of the others would be anywhere near as interesting a piece. That and the avp films are truly terrible...not to mention the fact that a $900 alien piece is one that will most likely be bought by only the harder core alien fans, as such it makes more sense to make on from a true Alien film rather than from a franchise mash-up.
Just my 2 cents.