In an article entitled "Landon Donovan Spikes MLS Ratings" Mediaweek notes:
Per Nielsen live-plus-same-day ratings data, the July 4 Galaxy-Seattle Sounders match drew 391,000 viewers, up 87 percent from a year-ago meeting between DC United and the Columbus Crew (209,000 viewers).
391,000 average viewers is not a large number, but if you remove various Wimbledon matches, it was a top 10 telecast for ESPN2 on the week. And consider that the show began at 10:30pm Eastern time.
If MLS matches could sustain that kind of viewership (a gigantic if), I'm sure ESPN's interest would be there to keep telecasting the matches.