The whole idea of presidents day is that we can’t get everyday of the year off and since had a number of presidents being honored, they just combined them into one holiday. The fact they were combined is not a way of taking anything away from the others who have been honored by the nation.
The reason king has his own day is because we obviously can't add him to presidents day because he wasn't one. If there was also a Malcolm x day and a Rosa Parks day then I would say fine combine them into civil rights day but there is only one holiday because there is only one man being honored...but if there were more holidays for civil rights leaders I would say it would be fine to combine them into civil rights day.