implementing geometry shaders is a massive undertaking that takes an expert in graphics code to write it for citra. heres the history behind it (as far as I remember it)
a long time ago ds81214 (sorry if i got the numbers in your name wrong) wrote the majority of the code to emulate geometry shaders in citra. but often the last stretch is the most challenging and for some reason that I don't know, they stopped working on it.
jayfoxrox picked up the work that they did and continued working towards getting it merged into the master branch. at one point, jayfoxrox felt pretty confident that it was ready for citra, and so we put the work into bleeding edge so we can have users test it for any issues. thats why really old bleeding edge builds have geometry shaders. well, like i said before, the last stretch is the most challenging, and jayfoxrox got busy with school and life, and the code eventually became incompatible with citra. When the code is incompatible, it takes a programmer to rework the code to make it compatible again, but jayfoxrox was busy.
recently, jayfoxrox had a little free time and energy, and started working on getting geometry shaders merged once again. He managed to get the first step (out of many) merged into citra, but sadly jayfoxrox got really sick and hasn't been able to finish it.
and thats where it is today. theres a few other people with the technical knowledge to finish it, but they either don't have the interest, or don't have the time to finish the project. Its a lot of work though, and its work that has to be done by a graphics code expert, so not just any one can do it.
it'll happen eventually, but like lg_0 said. it'll get merged when its ready.