Three reasons: I'm moving my personal infrastructure off of AWS, which means to keep DHB running I'd have to spend time and effort migrating it. (Why? Development is much faster on Cloudflare, and my AWS bill is ballooning. Rather than figure it out I'm closing my account.)
Secondly, it's not a viable business to ask users to supply a Coinbase Pro API key with transfer permission. Until Coinbase Pro offers more fine-grained API keys, this permission is just too powerful and puts a lot of liability on key holders.
Third, I can easily DCA using Calculon and a cron job and get the lower Coinbase Pro fees. A web UI isn't necessary.
Anyone who wants to talk trading, tweet at me! wagmi