Divisionals are always subject to the national events, especially when you start dealing with in-division or bordering nationals (e.g. you wouldn't want a divisional in Tulsa while there's a national in Topeka). Also, knowing the national schedule can help consolidate things. D5 has already announced that the Brainerd divisional will be held in conjunction with the national (divisional Thursday and Friday, national immediately thereafter). As tightly compacted as the nationals will be this year, they have to be known before you can start going down the line. The divisionals will likely start sooner where it's allowed, but that will be handled at the division level, not so much the national. At least a couple have announced tentative plans for their seasons since yesterday.
Also, it's not true that divisionals have a smaller car count. Generally, they're significantly larger than nationals from a participant perspective, so distancing in the pits and staging lanes could be more difficult, especially at the tracks that are limited in space.