Because they're not as safety-conscious. They also use those dangerous "belly belts" for infants on U.K. aircraft. They don't have a lot of experience with car seats on planes because many of their seats need a shoulder belt and are therefore not designed to be used on planes.

Remember too that while U.S. children are recommended to rear-face till age 2, at that age, not only are European children forward facing but in boosters (not recommended till age 4 in N. America).