Mostly for the very same reasons as the missing boattail.
The bubbles have lots of interior volume
The VW 1L and XL show they aren't dumb
They can do it - they just doubt the market is ready to accept their solutions.
VW / Audi tried with the Lupo and A2 3 Liter (per 100km) cars, and effectively failed as they didn't sell that many.
That surely makes any company cautious.
However, these cars are more in demand now, than they ever were when new !
VW is trying again, with the up! CNG (79 g/km CO2, that's Prius C territory).
A brave try in what's very much a niche market in most countries.
Fiat has the lowest overall CO2 output in Europe, yet the company is in shambles .
If they were, it wouldn't be so easy to ecomod them, would it ?