Securing mobile communications on a massive scale according to SCI government specifications does not happen overnight, and is not very easy. So unfortunately she has to use the channels provided, however clunky they are. I am sure Mrs. Clinton wouldn't hop into her car to visit a friend without gathering a full security detail. Yes this curtails her life, and yes it is inconvenient, but it is necessary just like securing her communications is!
It is like if I were working for a large bank, and I put in a back door VPN to the network since their VPN client only works on Windows, and I want to use Linux. Sure more convenient for me, but it is not my place to subvert security for my own personal convenience.