Fancy meat substitutes like quorn are expensive. TVP and tofu are dirt cheap. Going with vegetable sources of protein that make no attempt to directly replace meat, like rice and beans or peanut butter, is also cheap.

Basically what Alicorn said. People aren't necessarily satisfied with the cheap ones that are available - mimicking the exact mouthfeel and flavor of meat is difficult, and because many of the original meat substitutes are from Asia, they weren't common here until fairly recently Mock duck, aka Seitan (made from wheat gluten) is cheap, and very popular in Asia, but it seems to be a perennial also-ran in the US. Back during my veggie days I tried using it, only to find out I have a minor glutease deficiency (not full-on coeliac, but enough that seitan causes problems). It was by far the closest I've found to mimicking texture and mouthfeel for non-specific cuts of meat (as opposed to mimicking burgers or hot dogs or chicken nuggets or something); when prepared right it can be close to indistinguishable from meat.

Making good, cheap meat substitutes is a lot of work; Western would-be consumers often have high standards for them and aren't satisfied with the more-established forms, such as tofu, while new forms have substantial outlays for R&D (Quorn) and sometimes face regulatory hurdles or other barriers to acceptance (Quorn's initial attempt at a US release went very poorly). In the US, where meat production is directly subsidized, it's hard to compete anyway because there's lots of cheaper meat.