these religions are older so have had more time to spread and have more people.
main thing, i think, is that in teh previous generation or two there was a lot of english prachaar from buddh dharm. in sikhi, the english language prachaar is only starting in this generation with Tirath Singh Nirmala and Kamalpreet Singh pardesi and some others. We had english books before, but almost every single one of them was very shallow and not really spiritual but focused on outer rules etc which doesn't attract non-sikhs; or they were historical/political.
another factor, they were in a good position to take advantage of the explosion of interest in non-xtian dharm during the hippie eras. In sikhi, i think only Yogi Bhajan ji was properly positioned to take advantage of these times.
So... bottomline, it's mostly due to circumstance, but probably our community's focus on making their kids into moneymakers like doctors and pushing them away from non-making pastimes like prachaar has also contributed.