For goodness sake! The RPG Maker XP has been out, for what, more than 6 years now? I was kinda expecting that someone would make other tilesets based on other cultures by now, but so far... nope none except for VX with its Samurai pack.
That's because the moment a new maker comes out, EB generally abandons anything previous, and people follow suit. EB created the Samurai pack especially for VX, but had no interest in doing the same for old products. You are allowed to do your own edits to make them work for whatever maker you own, but you aren't allowed to ask for help, because it's a purchasable resource pack. It's shitty, I agree, but it's business.
However, every maker since time began has had the same theme/setting for the RTP. So getting the Samurai Pack (even if it was only formatted for VX) was really a shocker, since before then getting any sort of variable theme was unheard of, especially an add-on such as the Samurai.
We are seeing more packs released (through the RMW store) with a varying array of themes, but these aren't EB releases. These are made by 3rd party artists, and as you can tell by looking at them, these artists are primarily interested in VX / Ace format.
So if you're wanting more themes for XP, you're only real options are:
1. Use what's currently available.
2. Find, purchase, etc any non-XP sets and format them to work with XP.
3. Find an artist who still loves to create for XP (and probably be willing to pay them).