Over the last 10 years, 13 of 80 (16.3%) Elite Eight teams were from mid/low-major conferences (including UConn's 2014 appearance as part of the AAC).
2016 - 1 (Nova)
2015 - 1 (Gonzaga)
2014 - 2 (Dayton, UConn - AAC)
2013 - 1 (Wichita State)
2012 - 1 (Butler)
2011 - 2 (Butler, VCU)
2010 - 1 (Butler)
2009 - 0
2008 - 3 (Memphis, Xavier, Davidson)
2007 - 1 (Memphis)
Over the previous 10 years, 11 of 80 (13.8%) Elite Eight teams were from mid/low-major conferences
2006 - 2 (Memphis, George Mason)
2005 - 0
2004 - 2 (St. Joe's, Xavier)
2003 - 1 (Marquette)
2002 - 1 (Kent State)
2001 - 1 (Temple)
2000 - 1 (Tulsa)
1999 - 2 (Temple, Gonzaga)
1998 - 1 (Rhode Island)
1997 - 0
Over the previous 10 years, 7 of 80 (8.8%) were from mid/low-major conferences (take out Temple, and it was just 3 of 80)..
1996 - 1 (UMass)
1995 - 0
1994 - 0
1993 - 1 (Temple)
1992 - 2 (Cincy, Memphis St)
1991 - 1 (Temple)
1990 - 1 (Loyola Marymount)
1989 - 0
1988 - 1 (Temple)
1987 - 0
So the period from 2007-16 saw almost 2x as many mid/low-major teams reaching the Elite Eight as the period from 1987-96
Edit: I'm wondering if the exposure gained by expanded TV coverage over the last 20 years has helped some of the mid/low-major programs pull in recruits. In 1985, you'd be much much less likely to ever get on TV in a mid/low major conference than you are in 2017.