Not sure of the answer. Maybe because it isn't roundtripped by the designer? You have to be careful modifying the XSL by hand. Some stuff won't be roundtripped unless you use a Preserve Code Block (PCB). In general, PCBs are carcinogenic and I highly discourage you from using them, but sometimes you just gotta go Full Trust and sometimes you just need a good dose of PCB.
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra