To expand on what abraker said, scores on qualified maps used to kept as the map became ranked, they also used to give performance points which is important to note. Part of this change was to prevent players from being upset at losing pp (or just their score in general) if a map they set a score on was disqualified (this is why mappers would not DQ their maps).
It was decided to stop awarding pp for qualified maps, and at the same time, reset their leaderboards once they became ranked. From what I understand, the idea is keeping scores on qualified maps as they became ranked would require them to eventually give performance points. This would defeat the purpose of the change in the first place, as some people could still view a disqualification as "losing" pp they would have gotten had the map been ranked.
Try reading through
this thread, pretty much all points are covered at some point in there and it was a decision that was probably made with quite a bit of thought.
abraker edited their post while I was posting this