Linux entered the market at a time when paid alternatives were fully established and concentrated, servicing users/companies for years who became used to working with them. No paid txt2img offering comes anywhere close to market dominance for image generation. They don't offer anything that isn't available with free alternatives (they actually offer less) and are highly restrictive in comparison. Anyone who is doing anything beyond disguised DALLE/Imagen clients, has absolutely no incentives to use a paid service.