The reason why vegetatively reproducing crop plants are best suited for maintaining hybrid vigour is that:

Hybrid vigour is the tendency of a heterozygote individual to show qualities superior to those of both parents homozygous dominant or homozygous recessive genotype. In vegetative reproduction, a small part of the plant body develops into a new individual. As there is no recombination in vegetative reproduction, the plant does not loss of heterozygosity.