1-Smaller communities are more organized than majority, simple as that. Islamili community is not a wealthy community on average but they allow their members access to money,business and good jobs and hence consolidating the financial position of their community. Deubandis do this to greater extent (Especially the ones in tabligi jamat) so do shias. It's not related to all being super rich but the rich ones pulling poor ones out.
2-It's an old practice that religion be offered in exchange of food, wealth and security. For example, Missionaries used the same in Sub-continent and Africa. Agha Khan foundation did this to some extent in GB. And there seems no visible purpose that Ahmadies would do anything different with Canadian $$$.
3-Ahamdies are not the only one who make a contribution to the jamat fund but the others do this well. Ismailia, for example, pay a certain sum to their jamat. However, that money collectively is used to advance the interests of the jamat. No rocket science here.