Sh'liach Mitzvah money

The ADDeRabbi has an interesting post on Sh'liach mitzvah money, in which he notes its likely origin in the ma'amar on Pesachim 8a that "Shluchei mitzvah einan nizokin", messengers sent to do a mitzvah do not come to harm, but questions some of the methods in which this custom is observed nowadays, most notably how it is only applied in the case of people traveling to Eretz Yisroel.

In a comment on the post, I noted:
My impression was that, when it comes to the priorities of giving tzedakah, the aniyim of one's own community come before the aniyim of anywhere else, with the exception of aniyei E"Y, who are on the same level of priority. If this is true, a Jew in NY could use someone traveling to E"Y to be his shaliach to give tzedaka there, but if the traveler is going to Los Angeles, he could not do so, because the sender's obligation would be to distribute the money in his own town, rather than in another town.

Granted, I haven't heard of people nowadays being makpid not to support aniyim of cities in Chu"L outside their own, but perhaps this could still explain the reason why, when the "laws" of sh'liach mitzvah money were codified, only travelers to E"Y were included.

Alternatively, perhaps the practice of sh'liach mitzvah money was initiated at a time when the community there was experiencing significant financial struggles (i.e., any time before the late 1800s). If this is the case, perhaps the practice of sh'liach mitzvah money, which as you mentioned is only tangentially based on the gemara in Pesachim, was innovated specifically to save the Jews living in E"Y at the time.

In a later comment, I asked for the earliest mention of this practice in the halachic literature.

Several posters challenged my post's implication that one who goes against the order of priorities is doing something wrong. One noted that the order of priorities is not an order of precedence, per se, but rather is ultimately translated into a fractional disbursement of one's funds (and further noted that aniyei E"Y are on a second level between aniyei ircha and aniyim of other cities). Taking these into consideration, one can still posit that if one is sending money anywhere, it makes more sense to be sending it to E"Y than to a city in Chu"L.

Upon further thought, though, the practical origin of the practice suggested by my second answer seems to me to be more likely than the halachic origin suggested by my first answer (It seems to me that this is a k'lal which can often be successfully applied in understanding contemporary Jewish practices whose origins are unclear). Ben Bayit in a comment there noted that the Gra and Chida are recorded in the "pinkas Eretz Yisroel" kept by the Jewish community of Amsterdam to have been given funds for their personal use in traveling to (back to, for the Chida) E"Y and for disbursement in the community there.