There is probably a message limit because there is only a certain amount of storage space available to each member. If you don't delete your messages you will take up a LOT of space and memory. The limit is so that those who don't delete their messages, can't take up a bunch of space. It forces you to delete if you want to message people. So basically its so you don't use a lot of space and memory.
Once again, I'm not sure if that answered your question.