Because.... that would be a sane and inexpensive approach to things. Not the european way. Like IFR in airspace G - for decades the German ATC did everything to keep it forbidden, with SERA they should allow it, yet there is rumours that they will not.
And then technical incompetency - when the LBA (German FAA) found out that their Beechjet had GPS installed in the "depths of their FMSes" despite them not allowing the use of GPS in aircraft at all we all had a good laugh. (many moons ago)
Flying individually is seen more and more as a bad thing. Hence EASA-Ops with crazy unwarranted regulations/requirements for anything flying bigger than a Colibri.
OTOH, given the topography (clearance to the sides), I doubt that Trier could have an approach with a low enough minima. For this case the minima would most likely been above the fog. And CAT III with GPS (GBAS) - which would have been required here - at Trier won´t happen soon. Bremen/EDDW does trials with system destined to become the CAT II/III replacement
AFAIK.