something we missed out on the early part of this thread is the limits to the term 'popular' or what measures the 'popularity' of the fish.
'Pricey'-ness
In general,as marketing basics dictate, something 'popular' tends to be more expensive. the money factor palys a significant role in this discussion since costly fishes tend to be labeled as 'in demand'.
'Prettyness'
Does a fish viewed as pretty by the mainstream market earn the right to be called popular?well, sometimes but not necessarily. Location, species variation, and personal preference are points to consider. this is almost impossible to standardize as different people have different tastes.
The "in" factor
I think this is the term we should have used the first time. Somethin "in" automatically increases its price. Something "in" (gradually or sometimes, suddenly) becomes pretty. Unfortunately, a lot of hobbyists tend to follow trends.