Like in most sports, NASCAR seems to be a sport where the lower the number, the better you are.
Of course, this is not to say that high numbers are always bad. Sometimes you can find a few good one in the upper numbers.
Reasons may vary as to why they are up there. They may be newcomers to the game. They may be lower-budget teams, so they had to take whatever they could get. Maybe a driver has fallen out of favor, or is getting older.
Sometimes they make for some interesting driver/number combos that roar to the front and mix it up with the big boys.