The main purpose of a tongue is to assist in chewing food. Most vertebrates generally have one or several rows of teeth, a cavity behind those teeth, and a tongue for moving food around - pushing it into the teeth for further processing or back into the throat when it's ready for swallowing.
But this is not the only way. There are plenty of invertebrates out there that use an entirely different mechanism for chewing. Many arthropods, for instance, have small, leg-like structures around their mouths for moving food into the right place for mashing or tearing with their powerful mandibles. Since an insect's mouthparts are "open", unlike a vertebrate's, their "hands" can manipulate food as much as they like until it is ready to pass to the throat.
If a species has hands or hand-like structures, it doesn't really need a tongue - it can use its hands to manipulate food around the "chewing" organs instead. But the vertebrate jaw is built around the tongue - it's hard to use your hands to manipulate food inside your mouth. So your species would require a pretty compelling reason to lose their tongue over the course of evolution.
The only reasonable reason I can think of is that their world was inhabited by a parasite that infects the tongue specifically. (Sounds silly, but there actually is a creature that does this to fish. And replaces their tongue with its own body.) This could potentially lead to tongues growing smaller and eventually disappearing altogether.