Until Tony determined the scope of the Extremis project, specifically the number of people involved, he had to conserve his resources. With his house destroyed, Killian probably assumed Tony to be mostly out of the picture. A nuisance to be sure, but not much of a threat.
If Tony had called for the other suits as soon as he crashed in Kentucky, or when he assaulted the Mandarin's house, then he'd have remained a target of Killian and his troops, which would have made his investigations more difficult and would have resulted in a significant amount of collateral damage. Not to mention that that may have pushed Killian to forcibly create more body-bombs in an attempt to distract Tony.
Instead, Tony waited until Killian baited him, and when he could be reasonably sure that all or most of the Extremis subjects would be present. This enabled Tony to remove as many of the targets as possible at one time, while limiting collateral damage and civilian casualties.
Alternatively, JARVIS may not have been capable of controlling all the armors until that point in the movie. The destruction of Tony's house seemed to have had a severe deleterious effect on JARVIS' capabilities, requiring JARVIS to go offline for a large part of the movie. Even once JARVIS was back online to where he speak through the MK 42 armor, he still had issues with some of his systems (such as occasionally substituting an incorrect word at the end of sentences).
As such, to acquire enough processing power to be able to simultaneously coordinate all of the other 40 suits of armor could have takes JARVIS significant time, in activating and/or "borrowing" auxiliary systems and in restoring backup programs and data.