When 0x was conceived, there were far fewer crytocurrency exchanges than there are today. It aimed to solve a real problem that still exists in some ways.
Since 0x is a blockchain project, it has moved at a snail's pace compared to the multitude of centralized and web-based "decentralized" exchanges that have sprung up since. These newer exchanges have largely solved the needs identified by 0x.
There's still a big problem with many of the centralized exchanges that have sprung up: a bunch of them are exit-scamming, or otherwise have had incidents that make your funds inaccessible.
That's the reason to do this on the blockchain: you don't need to trust that the exchange operators will keep your funds safe, because they don't have your funds.
An even better solution is to transform exchanges into channel hubs, and settle trades instantly without ever touching blockchain. I.e. trade dynamic remains the same, but now each party holds a balance proof and redeemable onchain collateral.
Since 0x is a blockchain project, it has moved at a snail's pace compared to the multitude of centralized and web-based "decentralized" exchanges that have sprung up since. These newer exchanges have largely solved the needs identified by 0x.