all other organisms tend to increase their functionality/complexity , like secreting enzymes, developing protective coats around them, etc. But in case of viruses, they became more simpler and lost their functions rather than gaining new ones.
Well, it really all depends on the environment. There does seem to be a tendancy towards complexity, but this is due to available niches in the environment allowing for it. If the environment exhibits pressure towards simplicity, then that will happen too. For example, fish in caves that lost their eyes:
Presumably, the same goes for viruses.