I'm not sure there's an answer beyond "because sometimes 'は' is pronounced 'wa'."
Kindof like asking "why is 'c' sometimes pronounced like an 's' like in 'cistern.'"
"こんにちは" is another word in which "は" is pronounced "wa".
I'd imagine there are probably historical reasons, but I don't know them. I suspect the cases where "は" is pronounced "wa" are pretty rare and just have to be learned by wrote.