Conventions are excellent ways to gain momentum. Having it in June will make any momentum gained obsolete by November. Furthermore, far fewer people are interested in the race in June of an election year as opposed to august, September, and October.
Contribution limits. Whenever a contributor writes a check for $5,200 they're contributing $2,600 to the primary campaign and $2,600 to the general campaign. You can't move general election money into the primary, but you can carry primary money into the general.
You can only access the general fund once you are legally the nominee of your party. Holding the convention in June means the nominee can use the general fund throughout the crucial summer months.
You don't normally think of the Romney campaign as particularly strapped for cashed, but in the months leading up to the convention, they were facing serious financial issues.