If it was really that easy a decade ago why was there no marketing done to reach people like me?
Show me pricing lists and contracts from 10 years ago indicating it was cheap as Twilio was 2 years ago. I can't see anything at http://web.archive.org/web/*/http://voxeo.com :(
I suspect the infrastructure was there for the 'big boys' and that indie devs and 2-3 man shops who wanted this sort of functionality were ignored in favor of 'enterprise' development shops and projects.
OK - rereading your post here - yes, you got 'distracted' by big deals. But it's the focus on getting the basic fundamentals as easy for people to use as possible which has created the Twilio loyalty and fandom they have relative to voxeo/tropo.
Twilio has (imo) about a year to add some more features that people are asking for before people jump ship. They should not waste that goodwill and rest on their laurels. But it's going to be easier for them to grow their success with smaller shops and indies in to something larger as those smaller shops and projects grow to larger needs than it is for 'enterprise'-focused groups to prove to the smaller players that they 'get' the independent developer market.
You can suggest that this is 'just' marketing/evangelism. I suspect it's a bit more - a focused simplicity on getting a few core things down first rather than trying to offer multiple services on day one. That doesn't appeal to everyone, but I think it's helped more than hurt in Twilio's case. Simply by having more options up front you're forcing people to have to learn a lot more about stuff on your terms when making an evaluation. For example, Tropo's APIs - 'webapi' vs 'scripting'. Huh? What are the differences? I don't find an adequate explanation of the strengths/weaknesses of each model.
Both services have their place, but developing a rabid fanbase will serve Twilio better longer term as opposed to pure technical superiority.