Well two things before you bring in attach ratio, 1: it still depends on what titles were released with it at that time (on the 3DS in the same timeframe for example) and 2: look at the titles released in Japan, which sold the most and continue to sell the most. The 3DS for example had together with Monster Hunter, many games like Pokemon, Smash, Yokai watch on it released which sold big numbers over a pretty long stretch. Those factors can be taken into account. This MHW had, in my opinion, a clear and open way to have great legs in Japan.
On the contrary I think that is an advantage for 3DS games. Pokemon, Smash, Yokai, etc. will push HW and people don't just buy one game, more like 5-10 games eventually. So MH on 3DS got help from other SW. While right now PS4 is in a dead zone software wise after a decent string of releases.
Theoretically, if MHW released in Jan, then in March we have FFXVI, and in June DQXII then I think MHW would have better sales.