because of the same problem as other contracts the lack of trustworthy Oracles, you need some trusted third-party to confirm what happened on the real world.
For example: We want to prevent underage users and duplicates. We do that by manually screening IDs. Hard to do that on a decentralized system...
Another major issue: In sex work, clients present a significant threat. An abusive client needs to be reliably banned. Having them put up a deposit is just not a substitute. E-commerce do not have this as client only receives what seller sends and poses no risk to seller.
We from our central point-of-view can detect rings, ban money launderers and traffickers, investigate and appeal if needed. Decentralized system that adds such a feature is ripe for gaming.
And yes: Single point to make money, campaign, run ads, provide incentives (sign-up-friend, get-$50-credit) and organize labor...this is an important aspect as well.
Scamming was rife, but largely mitigated by the fact that the dealer cant effect their reviews and nor can the buyer. The dealer has the choice to not sell to you based on your history, the buyer has the choice to not buy from a dealer with a bad history.
Other than that, bitcoin had little to do with its security apart from providing anonymity. If it was possible to do so anonymously you could have just post them cash.