I think what had had a large negative impact on the Missouri Valley Conference's at-large chances came from the departures of Creighton (2013) and Wichita St (2017), which coincided with the switch to the NET system in 2018.
Over the 20 years up to Wichita St's departure, the MVC had 37 bids.... but 16 of them were from Creighton or Wichita St (10 auto-bids, 6 at-large). From 2014-2017 after Creighton left, the conference had 6 bids in 4 tournaments... and 4 were from Wichita St (2 auto, 2 at-large).
In 2018, the MVC moved forward without two of their best programs of the prior 20 years, and NET was introduced to assist with tournament selection. Sort of a perfect storm of the MVC conference SOS dropping at the same time that NET quadrants were introduced.
Since then, the MVC has had just 7 bids across 6 tournaments (1 at-large in 2021)... 3 from Drake, 3 from Loyola-Chicago, and 1 from Bradley. Those three schools had appeared in the NCAA tournament just twice in the prior 20 years (1 from Drake, 1 from Bradley).
Conference consolidation has been gradually plucking the better mid-major teams into stronger conferences, leaving behind an increase in weaker conferences in its wake.
Looking at the top 5 conferences (SEC, B1G, B12, ACC, BE), you see a lot of schools that came from mid-majors over the past 25 years: Butler, BYU, Cincinnati, Creighton, DePaul, Houston, Louisville, Marquette, SMU, TCU, UCF, Utah, Xavier. And the mid-majors that lost those schools pulled the better schools up from the next tier down (for example, C-USA was founded in 1995... not a single member from 2004 is left; the MWC grew out of the WAC in 1999, and will be unrecognizable in 2026)