I think because many people, myself included, are not too keen on non-transforming toys of transforming mecha, no matter how nicely articulated or how cheap they might be.
Plus, I have no interested in putting my Valks into funky break-dancing poses.
Many of us are also burnt out on VF-1 toys, due to the market being flooded by many different versions from Yamato, Bandai, Banpresto, Toynami and others over the past few years.
Despite all the above sentiments, I'd probably pick up a Revoltech YF-19 or YF-21 if they were not too overly stylised.
Graham