i would imagine that the stars of the 1991 series do not participate in the fest for monetary reasons on several levels.
mpi has 1,225 episodes of the original series to sell. the 1991 version has 12. it's doubtful anyone is ever going to make money on that and like it or not this is a business enterprise.
in terms of the actors onboard most who attend from the original series do so not only out of fan loyalty but to promote their own projects. kathryn leigh scott,lara parker,marie wallace,diana millay,david selby and so forth all sell books,cds and so forth based on their time on the program and assorted side projects but still market them under the dark shadows banner.
the stars of the 1991 series do not market themselves in the same way and there isn't the type of merchandise associated with the original series for them to promote.
it's likely not financially viable for anyone concerned for those actors to attend.