I know another sportsman within Australia who has been asked numerous times to sign and flatly refuses to do so. Based on what he earns he doesn't need the extra cash and just feels no need to sign cards.
Some players use management agencies, some manage themselves, some have overseas managers and some just don't want to sign (or their managers dont want them to sign) for another talent management agency who has a trading card licence.
TLA/ESP would have had to deal with probably 20 different mangers/agents/players to get the 40 or so players to agree to sign.
Would be like herding cats i assume!