Wait, when did Frobin become unpopular? Did I miss something? Or is it that just that after years of absence, they're not fresh on our minds anymore so people don't draw as much art of them anymore?
(being part of the fandom for over a decade, maybe I am able to share some observations)
FRobin never was and/or is never really unpopular. I dare to say it’s actually the opposite. FRobin is widely popular with a lot of One Piece fans, and a lot of One Piece fans support them. Which causes a very interesting problem... there is to all times only a small group of active fans.
With active I mean people who write fanfictions, draw fanart, do graphics, create blogs and themes and music lists and whatnot.
Only a small group has them as their OTP and lots and lots people have them as a side OTP.
Looking at the notes of this blog it is easy to see that a lot of people enjoy the pairing. They are Reblogging and liking a lot. But if you have a look at the Frobin tag, you’ll realise that only few post there. I’d dare to say that it might have become a little better the last few months but I’m not really sure.
2013-2014 had been years with lots of new fanworks, thanks to frobin week 2013 (49 pieces) and frobin fortnight 2014 (82 pieces).
Right now is an event #FrobinSummer2018, so far not a single piece was uploaded.
So, the problem is not that it is unpopular. It is invisible. If no one creates no one will find it and everyone will think they are alone, making them think it’s unpopular.
PS. the original posting is almost two years old (15. August 2016), which had been a year with almost none new fanworks. I’m also sure there are still people who think that FRobin is unpopular, thanks to the lack of fanworks.