Because unless he's a con man Mills is a *********** moron who wouldn't know remedial business sense if it jumped up and bit off his nose.
If he had a prototype capable of sustaining a stable reaction he should have been selling it as a heat source 20 years ago.
He is a medical doctor playing physicist on someone else's dime. Even if his science were right, the last 30 years are proof he is incapable of developing a working product.
The fatal and obvious flaw in your 'reasoning' is that you assume it is easy to produce a stable, continuous reaction of suitably high power.
Nobody has done this kind of work before. Mills is the first.