Honest question. Why is it looked at as SEC teams won't travel north for non-conference games? When was the last time a Big Ten team played a non-conference game in the south (non bowl)? Historically, the Big 10 has played more Pac 10/12 and Big 8/12 teams while the SEC plays ACC and Big 8/12. I hate the SEC hype as much as anyone, but this view is pretty skewed, IMO.
It's not as if the SEC is avoiding playing big non conference games. Louisville, North Carolina, Wisconsin, Arizona State, Texas Tech and Oklahoma were all opponents of SEC schools this year. Upcoming nonconference opponents for SEC teams:
Alabama: USC (2016), FSU (2017)
Arkansas: TCU (2016), TCU (2017)
Auburn: Clemson (2016), Clemson (2017)
florida: FSU (2016), Michigan, FSU (2017) - Michigan is first OOC game outside of Florida for uf since 90's
Georgia: North Carolina, Georgia Tech (2016), Notre Dame, Georgia Tech (2017)
LSU: Wisconsin (2016), BYU, Syracuse (2017)
Ole Miss: FSU (2016), Cal (2017)
Tennessee: Virginia Tech (2016), Georgia Tech (2017)
Texas A&M: UCLA (2016), UCLA (2017)
And remember, the last time an SEC/Big 10 series was scheduled (UGA/OSU for 2020), the Buckeyes were the ones that backed out.
That's a real cute list that I'm sure you spent a lot of time on, but I'm sure you had to notice that out of all those games, only 2 will be played north of the Mason-Dixon line (GA @ ND, LSU vs. WI).
The FACT is that the SEC is BY FAR the least likely conference to schedule a non-conference game in a Big Ten stadium, and it's not even close. When SEC teams actually play outside their home stadium, it is unlikely they will leave SEC country, and rarely even leave their home state. Alabama's visit to Happy Valley in '11 and LSU's trip to Green Bay next year are rare feats. The recent IN-MO series was scheduled way before MO joined the SEC, and let's not bother counting Vanderbilt.
When the SEC is put on a pedestal the way it is, and the Big Ten is criticized for its poor bowl performance vs. them, which all take place in SEC country, it is natural for Big Ten fans to want a shot at these prima donna programs in their home stadium (especially in November). Since it's common for Big Ten teams to travel across the country to play non-conference games, it's reasonable to conclude that it is the SEC that is giving the push-back preventing those types of series, not the Big Ten.
So, to answer the original question in your post, I guess the answer is "Because it's true and justified."