This is the announcement of them ending it back in 2018. https://worldofwarcraft.com/en-us/news/21655806/the-remote-auction-house-is-retiring
The old forums are gone now, but someone kindly snapshotted former CM Ythisens response about why.
The Web/Mobile Auction had a lot more issues with keeping it up than it was really worth for the small amount of players that actually used it. Trying to tackle and fix those issues (for transparency the majority were just exploits) just takes dev time away from working on new content. While it may change in the future, we currently at this time aren’t planning to bring this back.