It’s funny how most people think the Rama is a kula and want to be “machmir” like the S”A. For some people though, the S”A is a kula. They would be yotzei the halachah with the Rama but they want an excuse to get drunk so they are meikil like the S”A.
Well, because if you do like the rema, the mechaber holds you aren’t yotzei. But if you do like the mechaber, everyone holds you are yotzei.
Hence it is a chumrah.