|Missouri evening primrose, Oenothera missouriensis|
If you ask, "why do botanists study some plants and not others?" the answers are all very human. Does it grow where they work? Or at a spectacular location where they would like to work? Do they know the plant? That gives showy flowers an edge. Can it be grown easily? Having as many plants as you want allows much better experiments.
Evening primroses do most of those well, so they were noticed by botanists long ago. Which led to publications about their biology, and to more study.