Did what? The hardtop came out first and the targa came later.

The cars are built on a tube frame chassis so most of the body isn't structural. I'm guessing they thought the side windows woulda looked funky with the roof off, or something like that.

*edit*

Or rather, if you look at the roof of the hardtop it's kind of curved whereas the roof of the targa is pretty flat. The car wasn't originally designed as a targa but it was added later because the american market likes open cars. So I guess the roofline was reshaped to make the targa more feasible.