It's very curious to me that when I check out other tournaments around the country that MA2 is peopled by many more players with mid to low 800 ratings. And many don't have anyone playing in MA3 at all.
I think it very much depends on the depth of the "talent" at the top.
If MPO is populated by at least a handful of 990-1000+ rated players such that shooting 990+ is required to cash, you get less of the 940-970 players moving up to MPO...at least moving up quickly. If they're lingering in MA1, then the 910-930ish players are more apt to linger in MA2 until they're forced to. And so on down the line.
But if you've got an area where at best there's one or two 1000 rated guys and most of the MPO field is 950-980ish players, it trickles down. That's the case up here in the northeast. I'm a 960 rated player who at most tournaments is in contention to cash in MPO, even win on occasion. I'm rarely ever the lowest rated player in the division either. Plenty of 930-940 rated guys in MPO. When they're there, that opens MA1 to the 900-930 guys who might otherwise play MA2 (because they not only can contend, they can win MA1). And it trickles down that way, to the point that you have 800 rated players finishing at the top of MA3 on occasion.
The ratings breaks are set on a national/international level, where they fit best based on the total number of players in the PDGA. However, the breaks aren't always a universal fit to every region. Sometimes they fit perfectly to the player base, sometimes not.
I think if the PDGA forced players into their ratings-based division with no option to play up, there would be regions in which there would be little to no competition in MPO. Not saying that would be wrong, but the onus always seems to be to push everyone up the ranks regardless of rating if the top divisions would be thin otherwise.