i think the main problem is that you need the fastest response times possible to attract daytraders. You can't really manage that with a decentralized system at the moment.
Technically speaking, the way decentralized exchanges work, you will always have a high trade latency. Most short-term traders prefer to rely on a centralized entity for faster and more accurate trade execution, and since they are the ones providing liquidity, they are more attractive to occasional and long-term traders as well. Liquidity on exchanges is analogous to the network effect, it's hard to get started but then the value grows exponentially.