In some cases I say luck can play into it. But I think as time goes on the less likely "luck" becomes a factor. Luck was a factor for people like JennaMarbles and Hannah Hart who started near the dawn of YouTube and had their first video (uploaded as a joke) go somewhat viral. That can still happen but with how many people are trying to jump on the YouTube train, visibility for small or new YouTubers is shrinking. I think quality and strategy are honestly the largest factors that play into success at the current moment. You can have the perfect strategy to post videos but if your content sucks or your videos look like you filmed on a potato, no one is going to take you seriously and stick around.
And if your content is amazing but you do nothing in terms of promotion and marketing your gonig to see no change or growth.
I'm not one to talk since I'm still small, but I can say that since I started taking that understanding to heart, I've seen a huge difference in my engagement and views.