There are some pretty big differences in Waves and Bitshares.
Bitshares' derivatives work on free market principals and are fully decentralized. Waves derivatives are IOUs issues by a centralized entity.
The Waves orderbook is centralized, but Bitshares orderbook is decentralized.
It is expensive to create assets on Bitshares because the fee is high. This was originally meant to only attract premium assets, but kind of backfired IMO. Most people are probably using Eth and Waves because there is more money and hype with them, more ICO thirsty investors, and the aforementioned costs.
Yeah most of the eth and waves can always be used by most icos specially the earned bounties from campaigns, which can be used exactly for trading sites; well to mention about those ico participants. For now I started using bitshares and I find it not so comfortable with their set- up and the entire system of my mind was still on adjustment period; needs a lot of patience.