Why not just make it CGI (or FastCGI) compatible so you can test it locally or run it in other environments? At least for things that are actually generating responses rather than acting as middleware.

Oh, right, vendor lock-in.

We did not create our own engine to create "lock-in". On the contrary, it would be a huge win for us if we could simply run FastCGI or Node or whatever applications unmodified. We'd be able to onboard a lot more customers more quickly that way! Our product is our physical network (with hundreds of locations around the world), not our specific development environment.

But none of that tech scales well enough that we could run every app in every one of our locations at an affordable price. Meeting that goal required new tech. The best we could do is base it on open standard APIs (browser APIs, mostly), so a lot of code designed for browsers ports over nicely.

(I'm the lead engineer for Workers. These were my design choices, originally.)