Long story short — the Big 12's contracts with ESPN and Fox mandate those outlets up their per-season payments to the league if it adds members. Adding two teams would up the payments about $40 million per year; adding four teams would add $80 million per year through the life of the contract, which expires in 2025.
Because new members would get less than a full share of that money for several years — as was the case when Utah joined the Pac-12 — it would be a bonus to the 10 current Big 12 members, which would split the rest.