Because Dodos became extinct much later than dinosaurs.
Dodos became extinct at about 1662. Meanwhile, dinosaurs became extinct about 66 million years ago! This is about a 65,998,338 year gap. Should two creatures, one with fossils that haven't even fully developed, both be considered Fossilmon?
Also, it is more based off on an Ostrich, and has traits of a Moa, Kiwi and Emu.