If you are that enthralled with metric measurements, just refer to your 27" rims as ISO 630.

A major reason for the switch was that by the 70's the US was the only user of 27" wheels and the rest of the world had settled on 700C as it's standard road bike wheel size. European makers importing bikes here weren't going to change all of their frame and fork dimensions to accept 27" wheels so 700C got a foothold here as the size that defined a "good" bike.

My '85 Bridgestone 400 came with 27" wheels and was next to the bottom of their road bike line that year. The model above it, the 500, came with 700C wheels.

Maybe you can also ask why bicycles use metric fasteners instead of SAE. What's the matter with 10-32 bolts instead of M5x.8 or why should we need a 9mm wrench instead of 3/8"?