Because religion shouldn't be pressured upon players. Although it would certainly add an amusing sense of irony to Sunday games.
I can't help but feel that this thread - coming up with solutions to 'the haka' - sort of misses the point. Rugby is a game born in England, for most international matches between the UK, South Africa and the Pacific Islands the referees speak exclusively in English, the broadcasting of sport originates in England, the fact that national anthems are even sung in rugby matches is due to a Welsh response to the haka in 1905 - and has subsequently been practised by every national team pre-game. If any UK person cannot see the profound cultural impression that they have had on international rugby then they must be blind. With all this it is baffling that there is such resistance of other cultures practices in rugby to the point that there must be further UK representation - even if it requires re-purposing historical cultural practices to make a response. How about instead of challenging the haka, it is mandatory that Queen Elizabeth takes all kicks at goals? If responses are going to be arbitrary and contrived, they may as well at least be ridiculous.