Because then you don't get anything out of it. You don't get more money, you don't get better teams. You just get two extra mouths to feed. No point in doing that.
The SEC didn't add Missouri and A&M just to get bigger. They added them because those schools brought in a lot of revenue. Having St Louis and Kansas City in the footprint helped to get a larger TV contract. That also got the SEC additional subscribers to the network. Missouri's cable number isn't as small as you think. Only 20 states have 1 million+ subscribers. Three of those are already in the SEC's market. Of the remaining 17, most are inconveniently located for the SEC: Connecticut, Massachusetts, New Jersey, New York, Pennsylvania, Washington, Michigan, Illinois, California, Minnesota. That leaves you only seven others that are relatively close(and I'm sort of stretching with some of these): Ohio, Indiana, Maryland, North Carolina, Virginia, Texas, and Missouri. Out of that group, Texas and Missouri make the most sense. Plus, if you look at the pre-expansion SEC, only Florida, Georgia, and Tennessee have more subscribers. Missouri has more than South Carolina, Georgia, Alabama, Mississippi, Louisiana, Arkansas, or Kentucky. So yeah, adding 900,000 subscribers was a lot for the SEC.
http://www.tvb.org/media/file/Cable_UEs_by_State.pdf