Discreetness and mobile
Recruitment firms like discreet texting - yes they do! When Jay and I set up FP, our girlfriends both worked at recruitment agencies - getting hold of prospects privately is really important for them.
But to state the obvious more generally: mobiles are personal. They're owned by (or at least used near-exclusively by) individuals. Unlike email, which relies on being in a fixed location (at home or in the office) and kit which you're less likely to own (the office PC, their exchange server) and control, your mobile is YOU.