Why don’t more builders use epoxy for the lay-up? . The additional cost would be small compared to total for the total cost of the boat. Oyster HR etc lets see at least an epoxy option.
Not so small. On the 40ft that have that as an option, it costs about 9 000 €.
There is also another problem, Epoxy resins are sensitive to UV and that means that the gel-coat has to be colored and that costs almost as much. That's about 15 000 € on a 120 000 € boat. That's a lot of money.
All European boat manufacturers propose (as an option that almost everybody takes) a hull with a three coat epoxy barrier, for about 1500/2000 €. Most propose also as standard a vinilester outer layer on the hull composite.