Because it costs too much money and they don't want MLB TV if even one subscriber is forced to get it on a package when they didn't want the channel.
See Dish Network is a profitable company and they want to keep it that way. DirecTV has only recently gone into the black even though they have been in business for 2 years longer than Dish. Dish is still owned by the same guy that founded it. DirecTV has changed hands twice because it wasn't making money!
Accept it or not... that is the reason. $$$. Pure and simple. Dish wants things a la carte as much as possible or as little impact to the bottom line while trying to keep package prices in line.
DirecTV will pay ANY price to get exclusives at the cost of the bottom line if for no other reason than to keep that product away from its competitors. Dish would rather tow the line on prices and expenditures.
See ya
Tony