I think the WRU ideally want them all to be playing in Wales, but with a small pool to start with they can ill afford to ignore promising, emerging, Welsh qualified players in other leagues, mostly the English Premiership. So new players can be picked, but once they begin to figure more prominently they'll be under pressure, but no obligation under Gatland's law to move to one of the Welsh sides.
Equally, they fear losing their established, big stars if they deny them the earning and trophy winning opportunities that a move to England or France would offer. Hence, 60+ caps and you've earned that opportunity.