I have a hard time believing that it would be more profitable to hold more conventions. Logic dictates that they would make more profit by cramming as many people into one venue as possible. With less venues they get the same attendance and contributions, but have to pay less for rentals.
What I learned working in convention administration:
The dubs are not big givers, but they are good sheep - they will give you whatever you ask for. So, if the expenses are $10,000 for a congo, all you have to do is announce early in the convention that you've only collected $3,000 so far and there is a "deficit" of $7,000 and you will get at least that much more before it's over. The key to understanding this is that the $10,000 in "expenses" is really about $5,000 in actual expenses for the convention, with the rest in the form of pre-agreed donations to the Society (usually in the ($2-3,000 range, always based on what they sent last time), payments for Circuit and District Overseer car insurance, repair bills, apartment expenses related to those living quarters attached to various Kingdom Halls, and a host of other expenditures that are presented to the elders at their convention business meeting (and nobody ever votes "no").
So, yeah, the more venues there are, the greater the opportunity to guilt the friends into making up the prearranged "deficit." These guys are no fools when it comes to making money for Mother.