Many of the evolutionary developments in plants developed in the sporophyte life stage. Because increased fitness of these increasingly sporophyte-"dominant" plants would result in a greater survival and reproductive success, these plants became more dominant than the more-limited gametophyte-dominant early plants.
- In other words, natural selection of the fittest plants (which happen to have sporophyte traits increasingly adapated for living on land) likely led to the dominance in the sporophyte life stage.
I would suspect this is due to the evolutionary advantages that are associated with the sporophyte life stage in land plants. Multiple advantages developed in the sporophyte stage of plants as they evolved from simple gametophyte-dominant bryophyte ancestors to the vast array of much more successful and dominant ferns, gymnosperms, and angiosperms.
Shift from gametophyte-dominated life cycle (in bryophytes) to sporophyte-dominated life cycle. Source: Cengage Learning (2016)
Evolutionary developments that have occurred across almost 500 million years in land plants include the development of stomata, true leaves, pollen and seeds, and flowers and fruits. Each of these developments occurred in the sporophyte life stage.
Each of these evolutionary traits had significant effect on the success of the various plant types that possess them, as can be inferred from the increasing success and dominance of increasingly more evolved plant Divisions.
Each of these sporophyte-focused evolutionary traits increased the reproductive success of plants (therefore swaying increasing advantage toward plants possessing increasingly sporophyte-dominant life cycles) due to their ability to increase plant size, growth, and success living in arid (i.e., terrestrial) environments.
stomata allow for increased gas exchange with more limited water loss (note: these were necessary with the development of cuticles in sporophytes of early bryophytes as well)
vascular tissue2 allow plants to grow taller/bigger/more complex while still receiving water and nutrients from the soil
true leaves allow for greater energy capture and growth (and therefore increased competitive advantages)
pollen and seeds allow for greater dispersal and removes/limits the constraints of aridity on fertilization
flowers and fruits increase pollination and seed dispersal
Reducing the gametophyte stage to small cells within larger sporophyte plants allows this otherwise water-necessary process to occur in the dry conditions of living on land. As such, the pollen-bearing plants (i.e., gymnosperms and angiosperms) have been very successful in colonizing vast terrestrial ecosystems.
Another evolutionary advantage proposed, (e.g., see Bernstein et al. (1981)1 and Michod & Gayley (1992)3) is that the diploidy of the sporophyte stage allows masking of the expression of deleterious mutations through genetic complementation. From Wikipedia:
Thus if one of the parental genomes in the diploid cells contains mutations leading to defects in one or more gene products, these deficiencies could be compensated for by the other parental genome (which nevertheless may have its own defects in other genes). As the diploid phase was becoming predominant, the masking effect likely allowed genome size, and hence information content, to increase without the constraint of having to improve accuracy of replication. The opportunity to increase information content at low cost is advantageous because it permits new adaptations to be encoded.
- However, this proposal has been more recently challenged by Szövényi *et al. (2013)4 because evidence suggests that selection isn't more effective in haploid vs. diploid lifecycle phases of mosses and angiosperms.
For further reading:
1 Bernstein, H; Byers, GS; Michod, RE (1981). Evolution of sexual reproduction: Importance of DNA repair, complementation, and variation. The American Naturalist. 117 (4):537–549. doi:10.1086/283734
2 Lucas, W.J., Groover, A., Lichtenberger, R., Furuta, K., Yadav, S.R., Helariutta, Y., He, X.Q., Fukuda, H., Kang, J., Brady, S.M. and Patrick, J.W., 2013. The plant vascular system: evolution, development and functions F. Journal of integrative plant biology, 55(4):294-388.
3 Michod, RE; Gayley, TW (1992). Masking of mutations and the evolution of sex. The American Naturalist. 139 (4):706–734. doi:10.1086/285354
4 Szövényi, Péter; Ricca, Mariana; Hock, Zsófia; Shaw, Jonathan A.; Shimizu, Kentaro K. & Wagner, Andreas (2013). Selection is no more efficient in haploid than in diploid life stages of an angiosperm and a moss. Molecular Biology and Evolution. 30 (8): 1929–39. doi:10.1093/molbev/mst095