Because it's a really weird thing to do with your free time.
It isn't comparable to console games, or MMOs, or board games, or card games. No, it's a
totally unique, and decidedly odd, way of spending some hours.
"Let's play make believe" is just not going to sell well to most people. Not even most kids, beyond very early years.
Sure, it has dice and rules and numbers. But
the core of what a RPG is, is what prevents it being a popular pastime in the scheme of things. There is no getting around that.
But other niche hobbies survive. I'm sure this one can too. The internet is perhaps the main thing holding it all together: finding group members for tabletop play, and starting or joining online play. Also, discovering games that might better suit, reading reviews, discussing systems and campaigns and more...
I find it amusing that some people post about how whole sections of the internet are - in effect - bad for the hobby, and where do they choose to air such views, along with (sometimes) advice on how to fix everything? Ah yes, the very places they are criticising! These places where newcomers are almost universally welcomed and assisted with impressive promptness and courtesy, where "veterans" can talk about anything they want to, where - as is common throughout the internet - there's nearly always someone on hand willing and wanting to answer questions on anything they're familiar with. Then there are the myriad other sites (blogs, information centres, etc.) with everything you could hope for, and more

, freely available to peruse, download, do with as you see fit.
It occurs to me that, at times, we don't know how lucky we are.