I think most people play the story first. (Often alone, to enjoy it more) They then reach level 40-45. After the story they play expeditions/bounties. The level progress is relatively fast at the beginning. The match maker tries to pair you with a player in the same range of level (+/- 3 level or so I think). But when progress is fast (at first) it's harder to find a match. So often you will end up playing alone.

Long story short. If you reach higher levels like about 55 to 60 you should get (more/faster) people into your group, but there's no guarantie. It depends on time as well. In the evening it's more likely to find a group then in the early morning. That are my thoughts. Hope that helps.