Mexico used to pass out 15 day transit visas to people crossing their southern border. Then they looked the other way as do-gooders provided trucks and busses as well as fod and shelter along the way north. The unwritten deal was that these people would get into the USA within the 15 days. They used to do so. For some reason, Mexico is not pasing out these transit visas now.
“For some reason, Mexico is not pasing out these transit visas now.”
Once the US/Mexican trade deal is done, expect the old shenanigans to make a comeback.