He who pays the piper calls the tune...
The reason is simple. International rugby is the paymaster and funds the game at all levels in all rugby playing nations.
In many cases, the tour schedule has been negotiated years in advance, oblivious of the demands on any specific group of players in any one season. Twice a year, in June and November, teams trek to the opposite side of the world at the completion of their domestic season. If you want the presence of New Zealand, South Africa or Australia to boost the coffers with full houses in Lansdowne Road, then you must be prepared to reciprocate. Last weekend, all four home countries faced southern hemisphere opposition and were systematically defeated.