[Jon Stratis]

The "mandate" to do districts within some sort of regional border is logistical. When placing an event (whether it's a district or a regional), you need to ensure its properly located close to teams, that it serve the teams properly. Allowing teams to "opt in" or "opt out" makes doing that a whole lot harder, especially in the border areas. You end up with a chicken/egg scenario - you need to know which teams are in before you plan events, but teams want to know where events are going to be before they decide if they want in or not. So, you have to draw some border ahead of time. Having teams from relatively far away truck in creates even more problems, as you need to ensure two plays for everyone in your district, and you often don't have many extra plays to go around.

So, you draw a line and say "everyone on this side is in this district" and go from there. Often, it's easiest to draw the line at the state border, but that isn't a strict rule. I can picture a geographic situation that would encourage a state to split in half - for example, if there are two distinct clusters of teams a long way apart, or if there is a single large cluster with only a few outliers near the border of another state with a nearby large city.

The other benefit to going by state boundaries is support - many locations (like Michigan) receive support specific to their state. It would be difficult to include outside teams with the same level of support from the local FIRST organization if things like money or state competitions had to stop at the state line due to external reasons.