Because in most cases the leasing company that owns the solar panels will not transfer the lease to the new owner, they will only agree to sell them outright at some outrageous price(usually the cost of the panels when they were new, which usually has absolutely nothing to do with their current market value or in fact with the cost of a similar installation today).
In many locations solar add zero value to the house. So it is possible that you owe more on the house+panels than you can get. Most buyers do not know the worth of solar and so won't pay anything extra for a house that has them (overall, a few are willing to pay, but others will see it as something they will have to pay to remove). Even if you find the one buyer who understands the value, the bank do not understand how it will affect finances and so assume zero - backed up with there are so few buyers who care that they don't see many higher offers coming in for houses with solar.