I can not explain why. I can only guess from various news items I read.

In case of JAL and Hainan, I kind of think that only the four foundation members (AA, BA, CX and QF) can act as the sponsor. Any member airlines can nominee any other airlines but only the Big Four can sponse someone. If that makes sense?

The case of BA is interesting. Because IT located in India where some airline would put it as the region of South Asia and sometimes being consider as a sub-market for Middle-east/South-Asia circle. Thus BA sponsored IT instead of CX. I believe CX is restricted to Southeast Asia and North Asia market. So theoritically speaking, if Gulf Air want to join oneworld, it may be nomineed by AA but can only be sponsored by BA. If BA do not wish to sponse GF then GF will not be accepted. The same would apply to Jet Airways.

Just my two cents. I would like oneworld to explain the process someday.