EGG-ZACTLY! If she's fun to talk to, that alone could explain her popularity. People underestimate the value of that. When I was in my 20s (god I feel old saying $hit like that...lol), I used to go to strip clubs a lot (having a girlfriend who swung both ways sure helped too,heheheh).
What I learnt from those days is that, just like everything else, once the initial "wow factor" of (in this woman's case) her magnificent cleavage wears off, if she's about as interesting as a mule to talk to, her channel will fail.
If this woman has 20,000 subscribers, it's because she's fun, articulate and fun to talk to. Sure, her looks might bring 'em in, but it's who she is as a person that keeps them there and keeps them coming back.
Even if you're pretty, being no more interesting than a box of rocks doesn't do it for many people (except geologists, in that analogy, hah).
Looks do only get you so far, and while that may be fine for some, most people prefer something more substantially engaging, yes.
And yeah, if it were just as simple as "pretty girl with amble breasts", there's plenty of other means to see such things without having to spend any money other than your ISP and power cost.