[JoshTriplett]

https://en.wikipedia.org/wiki/Solar_panels_on_spacecraft#Ion... might be one reason.

That said, the ISS uses enough solar panels to generate 120kW, so it's certainly possible.

[magic5227]

"Other glass coverings, such as fused silica and lead glasses, may reduce this efficiency loss to less than 1% per year."

doesnt seem that bad actually. Perhaps the batteries would die too quickly being charged/discharged so often.