Same reason unix systems typically did the same with theirs (albeit on port 111)
Its not very useful if other machines can't talk to it.
Keep in mind, the decision to do this in Windows happened back when home computers were on dialup. (Even RFC 1918 and NAT were young at the time) The real question is why didn't Windows have a firewall that blocked it by default and the answer is that it has.... since XP SP2 was released in 2004.
And going a bit further back, this is probably one more reason Windows was successful. Networking "just worked". It was pretty neat (in the 90s) to just plug stuff in, turn it on, and hey, there's the other computers, and printers and so on.
