It is a tribute to those that served and also marks the end of WW1. So is seen as a happy occasion.

Memorial is a holiday to salute those who gave their lives.

I don't think I've ever heard Happy memorial day. Although they gave their lives for the freedoms of others I'd think they'd want us to happy for that.

And good point about Easter, applies to Merry Christmas also. I'd say the line above applies to them too.