I don't feel like I ever really see people gain popularity out of nowhere? The closest we get to that is someone getting promoted via a spotlight or similar, and they blow up, but it still takes a quality comic to keep those people, and it has it's own draw back with dead subs and what not. Never forget that some comics/creators already built their audience else where, so if they appear to blow up, it might actually just be an established creator changing platforms.
Quality of the art, consistent updates, tight story telling, popularity of genre, luck, and other difficult to measure things all play a part in how your comic does, but no one thing guarantees you anything.
I think it's easy to see subs just as numbers, and covet the big numbers you see other people have. No one will hate on you for wishing for that, but if you're not in a place yet where you can achieve those numbers, don't forget that the 10, 40, 100 subs you currently have are individual people that choose to read your comic. That is CRAZY AMAZING you TALENTED FOOL.
As far as tangible things you can do to help your comic along, besides drawing and writing your best, make sure you have an appealing thumbnail, take time to actually craft a good description/summary, and update in a consistent manner. Promoting here, advertising there, also good, but a lot of it comes down to skill, and skill grows with time and practice.