If we can be brutally honest here for a minute, a subscription is
usually earned if the viewer
truly enjoys the content they're watching and wish to see more from you. I kind of think of a subscriber as a fan. The YouTubers I'm subscribed to, I'm a really big fan or I've been inspired at one point and I enjoy
every single video uploaded.
These random faces commenting on your videos probably just happened to come across via. suggestions on the right along with every other video and moved on right after seeing yours. Chances are, the video was nice for the few minutes but they didn't take the time to check out the rest of your channel.
I comment on videos all the time but I'm not subscribed to more than half the videos I comment on. I just so happen to enjoy that one or handful of videos from them, but the rest of their content is just not that appealing to me. A YouTuber really has to earn my subscription.
I don't know what you do, I don't watch your videos and read your comments but if you're not yet, some things you can try is replying back to the comments (accordingly) with kind words and thanking them in return for taking the time to watch your video. I probably wouldn't ask people to subscribe to my channel, I always feel others would be more fond of a natural welcome rather than being asked or feeling forced; but if you feel asking people is the way to go then by all means, do so if it will work in your favor.