Comparing Latin American with US experiences the only reason seems to be lack of demand.Benedict Evans
made some good points about the mobile carriers, but they are not causes but consequences. In Latin America and Asia cellular calls were (and stil are) expensive, there is a huge part of the market that is pre-paid (because higher levels of poverty, so the majority of the people doesn't have access to credit) and in one moment one single call could cost you about 10% of your balance (from the most popular pre-paid card value, about US$5), so it came the SMS, it was cheap and at the beginnings some carriers provided the services for free, and people start using it to communicate as an alternative to the more expensive voice-calls.
On the other hand, cellular telephony had its boom during a economic bonanza in the US with cheap and easy credit, the majority of users were (and still are) post-paid, and unlimited calls plans were common, so why write when you can call? Also remember that full keyboard smartphones are kind of new, back then the most advanced phones didn't even have color screens, and they were usually slow and clumsy, so writing in your phone could be frustrating, also SMS was part of the GSM standard and back then there was TDMA, CDMA and some analog networks that didn't have a SMS standard defined, so SMS was not universal as it's today.
In most emergent economies SMS were adopted as a necessity and then became a trend, when it started the only texting happening was over ICQ, Compuserve and MS Messenger and it was considered as a necessary evil that would eventually be replaced by some low-cost voice service, but things turned the other way and texting became an alternative to voice not only for economic reasons but for all the advantages that we all now know and love, only then the US users found interest in using the short messages and then the mobile carriers found a reason to put money in the business.