Ok here we go:
Top engine builders: In particular who are you talking about? NHRA? NASCAR? It may have something to do with those engines are used above 3500 rpm and therefore make the variable duration lifters unnecessary.
The variable duration lifters are best used for street where you are at low and mid range rpms.
Now why don't OEM manufacturers use them:
I don't know for sure. I would think that they would be interested in them.
However, I don't think that Rhoades can handle that kind of volume. They are too small of a company to handle that responsibility, the liability could overwhelm a small company.
I worked on a V-6 engine line. We made 500,000 engines per year. This would mean 6 million lifters per year alone just for that one engine family, and there are other engines to supply.
Not to mention that one of our on line tests was very sensitive to "soft lifters". It would cause the engine to fail the test, even though it was a good engine. We would sometimes have to retest them 2, 3, or 4 times. That would back up the assembly line on days that we had many of them. I don't think that Rhoades can supply the kind of support needed to help root cause the problem like that, they are a family business. It was difficult to do with a large corporation that had manpower to support those issues.
There's alot of responsibility being an OEM supplier. It can overwhelm or put a small company out of business.