Several factors affect this:
(a) Several extinguisher companies have staff that are not conversant with current maintenance standards or technology. They don't try to recommend F Class extinguishers as they do not know about them
(b) It's easier to flog a few cheap Powder extinguishers & blankets!
(c) Wet Chemical extinguishers are extremely expensive compared to other types. Where you can be paying £40 - £60 for a 6 kg DP or 6 litre AFFF, a 6 litre Wwet Chem can be £120-£200. With the dearer companies where you pay £100 + for a standard ext, the Wet Chem then hits the £300. Even on local authority contracts with Chubb where you only pay £25 per extinguisher the Wet Chem is still £199.
(d) Ignorance amongst health and safety professionals, fire officers & some consultancies - if they are happy with a blanket and a DP, AFFF or CO2, why pay three times as much for an F class extinguisher?
(e) Lack of labelling on equipment - only Chubb blankets are marked with the fact they should only be used for pan fires up to 3 L or 300mm diameter & extinguishers don't have a "do not use on cooking oils" warning as EN3 doesn't recognise F class (causing terrible problems in Germany, where at first glance a WC ext seems to be a plain water (only the A-class pictogram and associated labelling) and thus totally wrong for the kitchen - they have to add a seperate yellow label)
Reasons C & D are most prevalent.
The situation is getting better, I'm seeing a gradual increase year on year in Wet Chem extinguishers or fixed systems, but It still leaves a lot to be desired.
Show a video of all the different types being used on a BS 7937 F class test fire and you get some quick conversions!