Did we consider that they added the Japanese Mahjong variation, popular/standard form in Japan, with the intent of appealing to a Chinese player base, who would likely be more interested in their own version? Nah, not really.
It's for the current playerbase, created by "just a few members of the dev team" (that's a quote from the digest for the latest live letter, so that's direct from them). They also had the following to say:
"We also wanted to give players globally a chance to experience a broad variety of types of gameplay. As we continue to create original content for FFXIV, we would also like to implement other various types of online game features not normally found in an MMORPG setting, so please look forward to them!
Originally we had planned to develop a poker game, but there are different legal rules with those kinds of games and how they are perceived in different countries, so we couldn’t implement poker as-is into FFXIV. We’re currently making some adjustments for this one!"
It's literally because they like Mahjong and decided to add it to the game because they want people to experience it and hopefully have fun learning and playing it.