Because most carriers in the US will be supporting SVDO, which requires the 512bit encryption for the use of an extra radio to enable 1X advanced and EVDO revision B, which Qualcomm has introduced in both software and hardware at towers, especially if you are in an area where Sprint has deployed Network Vision equipment.
The coding for CDMA as is being now required is more complicated due to SVDO, compared to the 2005 encryption Microsoft wants to use and push among CDMA carriers (and because that old standard is used in China, which their carriers have no immediate plans to have SVDO ready devices as Verizon and Sprint have, more obvious on Sprint since they have more than half of the current LTE devices supporting SVDO)
May sound like an excuse, but when most carriers follow Sprint's choices for WP devices is for a reason (whether they are MVNOs or Regional/Prepaid carriers with their own networks like US Cellular, Premier, C-Spire, Cricket, and MetroPCS), is obvious that they are following changes when it comes to Qualcomm's CDMA standards, for which Verizon and Sprint have a major input on in order to keep that technology up to date, and SVDO is possibly the biggest advance for CDMA in nearly seven years, which ends the GSM advantage of talk and surf at the same time, although at the cost of doubling encryption to make it possible.