Given his crippled and nerdy appearance in '99, I suspect Stark's spurning of him was just the last straw on the camel's back - a lifetime of pain, torment and humiliation (in short, being bullied) finally ignited on that rooftop. Tony was the personification of everyone who had made Killian's life a living hell up to that point, and hence made the natural target for his revenge.
At which point 5 years of speech and physical therapy made him a sexy media sensation. He could have done that any time...
What actually happened, according to script, is that Hansen called his number when Tony didn't sign on to her work.
If Hansen hadn't called, IM3 wouldn't have happened, and possibly IM1 and IM2, depending on how we interperate "10 rings."
Between the two of them they realized that they needed a "face" to promote the concept. Stupidly, they didn't go with the attractive woman, and wasted years creating an attractive man out of nothing. Cue suspension of disbelief.