The development of open software is based on volunteer financing
. Either someone devote their expertise and time, or someone pay for someone to do so.
As I understand it in the case regarding the Garmin plug
in, by pure chance, Dave got hold of a radar on a friends boat and made the initial effort to write software to replace the Garmin control console.
A volunteer effort financed Dave to get a Garmin unit to be able to write the "Garmin Radar Plugin"
It could just as well have been the Simrad BR24. But it was not.
So, either we have to accept that for now, the Garmin radar plugin is the first to be developed, get our hands dirty and write it ourself, pay someone to write it or hope that Dave writes the code in such a way that it is easily adopted for use with a BR24.
I am sure the interest for a BR24 radar plugin is as large as for a Garmin.
There is some work done already regarding the BR24 communications
protocol, and that protocol is published already.