> How common is it that these industrial/commercial buildings have roof-top solar? If not common, what stops it?

One factor is weight - a lot of low-cost industrial units may well need modifications to the roof to strengthen them to be able to take the additional weight of solar panels.

Certainly where I work (science research facility in the UK) our business has only been able to install roof-top solar on a comparatively small proportion of our total roof space because a large amount of the seemingly viable space would require substantial works to make the roof strong enough for solar.

In the US, there is a slowly-growing trend of covering parking lots with solar panels, rather than buildings. You can put up very cheap i-beans + corrugated metal structures with the added benefit of shading cars. It doesn't work everywhere, but in sunny - and especially dry - regions, it's great.

edit: I could see this working well in AUS and USA because we've got lots of space and sun. Might be harder in Europe and northern Asian regions

It’s becoming common but the current administration has been highly committed to preventing that because it cuts into their fossil fuel donors’ business.