Simple answer, when I bought the bike almost nobody had stock of any alternative items.
Crashbars is a good example, I wanted stainless which limits to BMW and Touratech. With no amount of effort could I even get an estimated delivery date on cashbars from Touratech. At that point the alternatives, that I was willing to fit, could also not supply.
Same for lights, only BMW could supply an integrated harness with switchgear, bracket and lights. Might have saved a few rand buying the lights elsewhere.
Bashplate, wanted aluminium for weight, could not get stock of an alternative.
Headlight Guard, don't like velcro strips and other such brackets on a R200k bike. The BMW option integrates well and is good quality.
I can go on but for me it's simple. Anything I fit needs to be of such workmanship that it looks factory fitted. In addition, the fitment cost was minimal since I ordered most of the gear with the bike.
The above list is the only BMW kit, the rest is Touratech, SW Motech, Rockfox etc.
Hope that answers the question.