It most likely is due to limited space. I wanted to point out that:
>Despite there being 151 Pokémon in Generation I, there are only 37 completely different cries in the Generation I games. However, different Pokémon's cries are different pitch or speed, have an echo, or disable one of the sound channels, making them sound different
Also, most of the similarities in cry are from Gen I Pokemon, with a few from Gen II. The more recent games haven't really been using the same cry for various Pokemon. In general, at least - I'm sure if I looked hard enough there would be examples.