At this point, marketing/mindshare is #1.

(That being said: they are priced competitively, do have a great API, can do things may others can't (ie MMS), and have great deliverability across carriers.)

One of the big barriers to entry in the space is spam whitelisting. Every time you send an SMS message, there's a chance the carrier flags it as spam and doesn't deliver it. Twilio and others try to give customers a higher chance of successful delivery. That comes with direct carrier relationships and stuff.