Velocity Deck - From the popularity of the Velocity RDAs sold by Avidvaper
Velocity V2 RDA
It has/had several appeals. A clever two post design that makes it very easy to build two coils on; a clever airflow adjustment mechanism; the V2 version supports squonking at a time when squonking was semi-new (I forget if the V1 did, but V2 came with a squonk pin option); and they were (probably still are) hand machined. They have a kind of good old fashion tough metal look (think old fashion Harley motorcycle appeal). Version 1 and 2 were around $100 when they came out, so targeted at the die-hard RDA users. It probably helped too that Rip Trippers, who was a major influencer, at the time version 1 came out gave it such high praises. Other influencers as well had high praises.
FWIW I still use my Velocity RDA (version 2). Built like a tank, it's one of those you expect to last a life time. You can still buy replacement o-rings, and screws, the parts that wear out.
p.s. and yes, the deck design is so simple to build on the design has been re-used in several other RDAs since.