Solar module efficiency is what, about 20% at best? Thermal is more like 60%. This means less roof area needed on a house that doesn't have a lot of solar exposure.
Then there's cost. With the thermal system I'm planning there will be 40gal of potentially very hot water mixed down to the (lower) maximum temperature of my 40gal electric water heater, which will again be mixed down to the maximum temperature allowed for domestic use. In effect the design will be one battery feeding into another. 'Seems cheaper than lithium batteries, and since this will be a passive system, no controller will be needed.