Historically, since 2002, more than three of every four at-large bids go to a team in one of the six power conferences. This means that, out of 36 at-large bids, on average 27 will go to teams in the power six, leaving 9 for, well, everyone else. By "everyone else" I include some fairly tough conferences like the Atlantic 10, the American, and the Mountain West.
I bring this up because over the past four years this disparity is only increasing. Since 2014, when "just" 72% went to the big six, the percentage has not gone below 81%. In 2017 it was 89% and this year 86%. So out of 72 at-large bids handed out over the last two years, only 9 went to teams outside of the big six.
First, realignment and other defections gutted some of the stronger mid major conferences. Conference USA lost four programs to the Big East in the mid '00s, and we've seen teams move up from the Colonial, Horizon and Missouri Valley. As the power conferences grew, they took some of the best teams from the stronger mid major schools.
Also, the very criteria the committee must use to choose teams clearly puts mid majors at a disadvantage. They want to see a strong schedule and wins against good teams. Yet they tell the power schools the same thing, so the power schools schedule against each other (for the most part) and the mids are usually left with scraps. St. Mary's is a great example. Their only really strong opponent, in or out of conference, was Gonzaga. I don't know if this was by design or not. Some schools do deliberately schedule lighter, but doing so at St. Mary's level will get you an NIT bid unless you just about win out.
I'd argue that the new quadrant system (which definitely has its merits) helps to create a feedback loop that helps the weaker-link teams in the stronger conferences. Baylor and Oklahoma State, Syracuse and Louisville and Notre Dame all played in the two strongest conferences. Granted, only 'Cuse got in from that group. But they all got large shares of "Quadrant 1" or "Quadrant 1 and 2" games, which put them solidly in front of teams like St. Mary's and MTSU (and nearly helped them knock St. Bonaventure out as well!).
The Atlantic 10 still pretty reliably gets 2-3 at-large bids, as does the American. The Mountain West has its moments, but even that conference only had one at-large bid over the last three years. Conferences do have down times, both mids and majors. The Pac 12 got just one at-large twice in three years in the early part of this decade.
But some are vanishing from the at-large discussion all together. Conference USA couldn't get one despite Middle Tennessee's great season. The WAC hasn't received one since 2010, the Horizon since 2009, and the Colonial since 2011. We're getting to the point where the A-10 and the American will get a couple and maybe the MW or WCC can snag one, but nobody else is even in the discussion. Monmouth had back to back great seasons in the MAAC and even had a couple of quality wins in there, but didn't get a sniff for a bid.
Can anything be done? Maybe the question should be "SHOULD" anything be done. Teams that play tough schedules either in or out of conference (or both) should be rewarded. But how do you reward a team that does that and goes 4-9 against those teams, vs a team that didn't (or couldn't) but won 27 games against the teams it DID play? I realize that ten years ago we had a run where over a three year period just 16 out of 102 at-large bids went outside the power six... so this has happened before and we've seen it bounce back.
But you also don't want this to be the Power Six Invitational, with a handful of power 7 and 8 teams plus a bunch of low-level AQ's, either. Do you? Because we're headed in that direction. Before 2010, 10 different conferences got at least one at-large bid at some point. Since 2014 that's down to 5... and over the last three years, only three non-power conferences received at-large bids each season: the American and A-10 all three years, and the MW, MVC and WCC taking turns. Meanwhile, 30+ schools from the big six are getting the rest of the at-larges. More than half of all power six schools have received either an AQ or at-large bid in each of the past three years. Not that it wasn't pretty close to that before.
Let's take a look at the non-AQ teams in the NIT. There were 21 non-AQ teams. Of whom 6 were outside the power six conferences. A little more than 25% . Two Conference USA and WCC, and one each from the MW and American. So 15 of 21 of these guys are from power conferences. Or in the NIT + NCAA, 46 out of 57 possible at-large berths were given to six power conferences, and combined only 12 out of 32 conferences (including the big 6) received at-large bids. This is what an expanded tournament would look like... maybe also with a few more AQ's for the little guys, which looks great until you see that they have to step on each other to earn the right to play those 1 and 2 seeds.
Should we care? Maybe it's silly to quibble about, what, four or five bids. Maybe we should want to see the best teams from the best conferences go head to head. And the revenue generated is mostly because of those teams at the top, I'm sure. But this is a thing. If you're a mid-major fan, increasingly you know that your team has to win its conference tournament or it's not dancing. The "middle class" that used to comprise maybe 12 or so conferences that occasionally got an at-large has shrunk to 2 or 3. The stronger schools have defected to bolster the shrinking middle or expanding upper class, and the lower class of about 22 or so conferences seems to have no hope of being anything more than a one-bid league. Yes, there's always been several that will always be just that... but there's no reason to expand that club any more than we have to.