Because Shinobi developer Griptonite Games was trying to develop one. Pitch art leaked out for Dinosaurs vs. Alamo from a former artist.
Another game brewing inside Griptonite has a lady strapped to a bomb.
Most intriguing is Griptonite also had a Tomb Raider pitch. This game appears to be tied to Lara Croft and the Guardian of Light. Since Griptonite makes a number of handheld versions, maybe this was a handheld version that never took off?