It's likely due to the limited availability of the HR44 and the extra cost to Directv for the swm16. They aren't cheap.
still frustrating to people like myself who already have the swm16. i have a 34, 21 and 22, so i'm only using 9 tuners on my swm16. i would love to swap that 21 for a 34/44 and be able to have 12 tuners. hopefully it won't be too long before they allow it.