Just a quick heads up that roll20 release their quarterly report on the games being ran on their site and Basic Fantasy is ranked 18 out of 65 listed. There are a few listings that are more of a grouping than actual system listing such as "Others" and "Board games".
For those that are active on roll20, congrats on making the system active enough to get listed.
I've played 5e and it's good at getting you going really quickly. Characters start out with quite a bit of strengths behind them and a lot of the rules makes it easy to play fast. Some things like advantage/disadvantage make it easy to modify the situation, either as rules suggest or DM wishes. Character creation also gives you a TON of options, so you can create your desired character pretty easily; the development is done for you. (Whether that's something you want or not is a different question, but it does speed things along.)
That all being said I'd rather have a nice quick character creation (which BFRPG makes really easy and simple), straightforward rules for combat (so I don't have to figure out if a bonus action needs to happen before or after some attack), etc. I'd also like to be able to just join a game quickly instead of spending a whole session JUST figuring out the process and the details.
I'd think BFRPG is popular on Roll20 beacuse it's easy to get a game going for a one-shot or single-adventure campaign. What's easier, reading an 11-page PDF (free on internet) which, assuming you know some basic table-top RPG ideas, is very quick to digest, or reading a 200 page book you'd have to order online. (Yes, I know there's a quickstart guide for 5e, but I had to read that for an hour to understand it.)
I play in a 5E game DM'ed by a friend. It's definitely a cool system, but it can definitely get bogged down sometimes. When characters have too many special skills (we're level 4), and you also have actions, and bonus actions, and movement, and etc., well... It kills a bit of the quickness of the system in my opinion.
My problem with 5e is that I feel they tried to put every option of every edition available in some way or other. It makes a lot of customization options for characters, but it makes the system lack any personnality.