Seat of my pants answer: because SSDs store data after the power is turned off.
Regular system ram places a tiny electrical charge in a particular place on the chip. That charge gets continually refreshed as long as power is supplied to the chop. Once power is removed, the charge dissipates. I have heard of forensic techniques that can read from memory some minutes after a system is turned off, but in general I think the charge (and hence the bit) disappears virtually instantaneously.
The memory in SSDs persists after power is turned off. That means that instead of just storing a tiny electrical charge, the chip has to actually physically modify the write location. Thus it makes sense that this could only be done a fixed number of times before the spot wears out. SSDs emplyo many tricks such as write-leveling to spread the writes uniformly across the chip so that one particular spot doesn't get used up more quickly than others.
I guess I kinda think of this as the difference between a capacitor and a battery, I'll be interested to see how others explain this.