Why all the mattress sales on Memorial Day?

Any holiday will do.  There are mattress sales for Valentine's Day and Halloween.

As PGroot said, almost every holiday is declared a reason to shop.  There may be a little rhyme to their reason, though.  Memorial Day falls near the end of Spring, a time when people traditionally do a lot of house cleaning and renovating for Summer.  This may be when people take serious notice of the condition of their beds.

This is, also, the time of year when bed manufacturers start introducing their new models.  So, retailers want to move out as many of the old units as possible.