portaudio is significantly easier to implement support for. this seems to be the main reason it is used. i am working with another well known audio software company and when faced with implementing windows support as part of their own cross-platform efforts, they decided to use portaudio for windows rather than try to code ASIO support directly (they had already tamed coreaudio to their satisfaction). it must be possible, but apparently aspects of it appear daunting.

in addition to ASIO, "modern" native windows APIs (WaveRT and WDM) are capable of providing the same low latency path that ASIO does. i don't know how many h/w makers actually provide drivers usable with these newer APIs, though.