I skimmed https://wikipedia.org/wiki/Decimal_separator but still don't understand. Why does this difference exist? Also, why did the conflict eventually settle into something between full stops and commas? What stopped other symbols from continued usage like bars or underscores?

It seems weird that a system would eventually settle on just full stops and commas, yet not settle on where to put them. If your system is going to converge strongly on two symbols, finish the job!