The President gets to decide where the convention is held, but they
don't always choose their home town. ( example, Phoenix in 2011) Keep in mind, the IAMPETH convention is held every year during peak vacation season. That means, the president needs to find a hotel that has the room to house everyone, classes and can provide an opening and closing banquet. They are usually looking for a combination of good quality/price, as well as nearby activities for the convention.
In years past, the following convention was announced at the closing banquet. I do believe the board voted last year to announce earlier, so that people can plan ahead and arrange for traveling.
It also helps to have a local guild that can help with set-up, planning and making goodie bags, among other things.
It's an experience like no other, and I highly recommend it to any calligrapher, of any level. Get to at least one, and it will really transform you, and your craft.