Because Reno never appears in KHI or KHII. Therefore, his persona does not yet exist. Also, if we follow the rules of OXIII naming, Axel and Reno are not anagrams with an X added. There's no other name that Reno went by in the original FFVII, so there's no reason to call him anything else.
"Ohh, but Squall changes his name!"
If you thought that, then you truly are hopeless, and should stop reading, lest your mind be overtaken by enlightenment and truth of situation.
Squall can be explained because it's a throwback to Leon of FFIIj, and also is a symbol of his abandoning his old name for not being able (at the time) of stopping the Heartless from overtaking Hollow Bastion, forcing him and Cid to flee to Traverse Town. He says so when you visit Traverse Town.
Also, this can easily be attributed to Nomura's uncreativity when it comes to character design.
Before you go any further, no, Axel is not Riku's Nobody, Riku and Sora are not gay for each other, and Sora and Riku both have parents. Disney doesn't do homo.







