What was The Syrians communities Rabbis motivation For disallowing conversions and when did it happen and did any other Sephardi communities join in this decision later on?
When their community first started to take root in the US many of their men began to date gentile women. This lead naturally of course to them wanting to marry these women. However given the strong social stigma and taboo associated with intermarriage (not to mention the halachic issur) many of them preferred to encourage their romantic partners to convert to Judaism. This however is a halachically dubious practice (see Hilchos Issurei Biah 13:14). Accordingly the community (primarily under the tutelage of Rabbi Kassin) enacted the "Edict." Here is a copy of a letter that Rabbi Saul J. Kassin issued to his community re-affirming the original edict: Rabbi's Message. As far as I am aware no other communities have joined in on this ban.
According to the Mahzor Aram Soba of 1527 and 1560, conversions were normative, but rare, as there are blessings in the Mahzor on the rituals of conversions. However, in the early twentieth century the Syrian Jewish communities of New York and Buenos Aires adopted rulings designed to discourage intermarriage. The communities would not normally carry out conversions to Judaism, particularly where the conversion is suspected of being for the sake of marriage, or accept such converts from other communities, or the children of mixed marriages or marriages involving such converts.
The ban is popularly known within the Syrian community as the "edict" or "proclamation" (in Hebrew, takkanah). Every twenty years or so, the edict is reaffirmed by all leaders and rabbis of the community, often with extra clauses. A full list is as follows:
Buenos Aires, 1927 (R. David Setton) New York, 1935 (Hacham Hayim Tawil) New York, 1946 "Clarification" New York, 1972 "Affirmation" New York, 1984 "Reaffirmation" New York, 2006 "Reaffirmation"