So SMS itself is actually data included in "overhead" signals that phones use to stay connected to the network in the first place. It doesn't work for landlines because there's no equivalent process for them.
That said, I think landlines are increasingly VOIP, which is to say, they use STUN/TURN, which is yet again a different technology from what cell phones do.
At this point the main impediment to supporting text messaging on VOIP or landline phones is that the equipment isn't made to handle it, so you're in a chicken/egg situation. If you create the tech but the phones don't exist to use it, it's worthless. If you start adding it to phones but the tech isn't present on people's services, they'll wonder why, or even avoid buying phones with such a useless feature.
Ultimately, people probably just aren't clamoring for it, since everyone's got cell phones now.
