My guess is that one problem with FLARM is that it is not available in a certified form, and there is a limit to how much velcro-attached electronics, powered from the cigar lighter, the owner of a nice plane is going to put in.
Even fitting proper aircraft power connectors for portable devices is a bit of a grey area...
The £1000+ ZAON unit which gives you some kind of directional information is a pretty tacky installation, with the cigar lighter lead running right across your instrument panel.
A saleable FLARM unit would need to interface to panel mounted MFDs or whatever. And then it wouldn't be £500 but £5000, plus aerials, plus installation, plus certification.........