Market driver is one thing, but you have to also take into account the cost of providing per event billing for SMSes. Sending even a kilobyte (while SMS is roughly 160 bytes) of data over ISP link just adds to total volume used this month. But in case of SMS it must be recorded who, when, to whom, etc. and then usually presented on the bill (be it paper or electronic). In the mean time you have to store it somewhere and process it through billing systems. Not to mention enhance the billing interface capacity of SMSC handling the traffic.