You have it backwards. You seem to think pro bike teams are looking to purchase frames from suppliers, and all the supplier needs to do is offer a decent discount. It's the other way around - most pro-tour bike teams require payment from the supplier if they are to display decals, and needless to say the gear will have to be supplied without cost.
You will often see a rider on a certain team has insisted on bringing his favorite saddle, which will then have all decals and logs removed if the supplier doesn't want to pay, or the team already has a primary supplier for that part. You will also see them removing logos from tubular tires made specifically for the cobbles, where most the big-shop tire sponsors do not have a suitable tire. As well, you may even see a team using old frames from a competing brand for a cobble stage where the current frame sponsor did not supply a suitable frame - in this case, the frame will either be rebranded or have branding removed.
In short, I think the lack of diversity you see is more a fault of the sport and the economics of it, not the fault of niche frame manufactures. Big high-revenue bike companies can afford to bet on multiple teams, while betting on one team may be a too risky proposition for a niche player (especially a team they can afford to sponsor).