It's probably more efficient to replace the entire roof at once, and insurers would almost certainly prefer the entire roof to be the same age, but asphalt does have some advantages for situations in which nearly every storm tears up just a few shingles.
More likely a glass roof is a net negative that has to be offset by the solar cells, making them less cost effective than otherwise.
or is it that such a cover gets dirty and not-so-transparent faster than the original top layer?
I'm guessing this must be something already being considered, so what am I missing?
Asphalt roofs are inexpensive. Given that hail-damaged asphalt roofs keep getting replaced with asphalt roofs, I have to think it's some kind of market/regulation/short-term-profit thing.