One reason is coming up with the capital cost, up-front. If you have the money sitting available, it's one thing, but if you have to borrow, the calculation changes, as does the possible availability of getting the loan at all, in the current climate.
I looked at it a couple of years back, when admittedly it was a LOT more expensive, but decided against because we are planning to move. So, if we installed, we've either got the cost of un-installing, moving and re-installing, or have to find a buyer prepared to repay the £13k installation cost, in the price of the house. So it boiled down to a £13k punt on being able to add £13k-ish to the house value, and that was a punt we weren't prepared to take.
Also, at the time, it was known the FiT was due to drop, but system costs were also dropping. So, it's also a £13k punt that the return on investment at the old FiT rates was better than the RoT would be at a lower FiT, but much lower capital cost. Now, MAYBE, the best deal was to pay more, earlier, and get the higher FiT, but betting that £13k against a likely much lower capital cost a couple of years later is, still, a punt.
Add there's opportunity cost. If I spent £13k then, or £6k now, that's £13k/£6k on which I won't get a return, potentially for a lot more than 20 years. If I buy the solar, the money is gone, and all I get is the return, in FiT, and lower electricity costs. If I put that money in, say, 5 year bonds, and reinvest on maturity, I get a return on that £13k, or £6k, indefinitely, and I still have the capital in the event I need it for something else.
So, if it was £13k 2 years ago, and £6k, maybe I'll do it in another 2 years, at the new house, when it costs £3k, and I get lower electricity costs and still have that £10k difference, that's been earning in the meantime. and no worries over selling the house and whether I'll get the money back in the house price. The lower electricity costs, and the return from the FiT are not the only factors, and indeed, even the money is not the only factor. And indeed, not all the considerations are monetary.