We don't know what exactly happens sia is a very old coin and why the coin is not supported for different Ledger wallets. There are many new other coins which are being supported in different ledger wallets and why sia is not in those lists.
I actually was just recently noticing this which is weird. Siacoin is a very old coin which had some great success on exchanges early on. Wondering what happened this coin actually made me some good money trading
Sia is supported on the Ledger. It is not user friendly yet, but possible as outlined in these instructions now: