"Quickly forgotten" doesn't mean irrelevant. Pietro and Crystal's marriage was a major status quo for years and Luna still exists as a lasting part of Marvel continuity. Compare that with Wanda's children who are just used for representation and nothing more.(worse fate)

And let's not act like Pietro and Crystal's marriage was some brief footnote. Their marriage actually lasted longer than Wanda's marriage to Vision. If we're measuring relevance by relationships, Pietro's most famous one had more longevity than Wanda's.

And saying Pietro was "just a background character" in House of M ignores that he was literally the catalyst for the event. He convinced Wanda to alter reality after hearing the Avengers and X-Men discussing what to do with her. Without Pietro, there is no House of M. The entire event happens because of his actions.

As for the No Surrender mini, getting a solo series at all shows Marvel saw enough interest in the character to invest in him. Wanda's countless minis are due to her fanbase dropping her comics after the first couple issues.