I can't point out one thing that made them popular, and probably there is not a single aspect, feature, offer or else that can make a broker popular overnight.
I think that the fact that they have been in the business for quite a long time, and the experience they gained made them aware of what traders want and need. They started providing exactly this, through their wide selection of account types, innovative trading conditions, flawless processing of payments, excellent support etc.

This is a great question, and good that you have noticed :)
I am sure their competitors study closely their recent success.
Although IronFx has been in the business for a long time, only in the last couple of years, are they being regarded as a top-tier brokerage.
What made them popular?
Probably, it's trading terms and the creation of a pretty long list of account types with innovative and unique trading conditions. Trading with zero spread is indeed a new and rare concept and you can hardly find any other broker with a similar offering.
Here I would mention their competitions. There are very generous awards and this makes them popular among traders who like to engage in competition.
Also, they improved their reputation by far.
So, a combination of various improvements is the reason for their success.