Ham relay towers are small and simple enough that they can be made portable and not subject to some of the location restrictions as cell phone towers. It takes a bit of planning and work to replace a cell phone tower but a ham repeater that sitting in donated space has to be made to be easily removed in case they can't use that space anymore so its easy for someone to grab one and haul it down to the outskirts of a disaster area.
Part of the reason I got into preparedness was from a job where I would install the phone and computer systems used for e-911 and computer aided dispatching. I realized that the 911 operator and police are just like any other job, you have a few that actually are good employees, some that are average and some that are just there taking space. So don't get me wrong and think I'm down on any profession but since you can't choose whom answers that 911 phone your take your chances of getting one of the not so good ones. I decided then I want to have enough tools and gear and supplies should I need to rescue myself but also have to hopefully never have to.