But how can a map become popular if it's not played anywhere?
Well, for most maps, including some really good ones - they just don't. The problem is... uhm, supply and demand. Warcraft 3 is already quite a niche game, melee isn't that popular as a game mode and melee players often prefer using maps they already know and like. In other words, while some do people play Warcraf 3 melee and there are some tournaments / ladders for it, the demand for new melee maps isn't high.
As for supply... Well, the game is old. The community had plenty of time to create a lot of really good melee maps that one could choose from. Also, melee maps don't have a lot of room for innovation or creativity, i.e. it's really hard to make a melee map that brings something new to the table, while also being considered balanced, a.k.a. it's hard to make your map stand out in any way. And if your map is just one of many, it's not likely to be picked.
Look, I have no joy in saying this - I've seen your map, it seems like you really put a lot of work into it and I understand you'd love to see people play it, but the reality is that if you want to make melee maps, the most likely outcome is that you're not going to find some incredible success. Of course, you can try reaching out to various tournaments or ladders, but I'd advise you to not get your hopes up too high.