Any idea how its compares to tRPC and oRPC? Wish all such project always had section of 'Why' and explained why it was needed and what it solved that other projects didn't.

I haven't actually used tRPC nor oRPC.

But my understanding is that neither of them support object-capabilities nor promise pipelining. These are the killer features of Cap'n Web (and Cap'n Proto), which the blog post describes at length.