Why is a transplant of fetal tissue less likely provoke an immune rejection response than tissue of an adult?

More Complicated. An adult will reject fetal tissue from a stranger without immunosuppression. From animal experiments we know that the fetus will not reject organs or tissue before it has a mature immune system that usually occurs during the first year of life. A mother does not reject her child's tissues in utero--with half of the genome coming from the father. The reasons for this have not been elucidated.
Less differentiated. I assume you are asking about embryonic, or fetal stem cells. They are less-differentiated. This means that they haven't been programmed to turn into a specific type of cell. Without this programming, they don't yet have immune complexes on their cell surface. Without immune complexes/identifiers, they cannot be attacked by a host.
Fetal cells... ...Are "immunologically naive" - that is, they have never had to develop an immune response against any other substance, and so are less likely to react against the body into which they are transplanted (so called "graft vs host" disease).