Of course, if you’re speaking about the Tyrells specifically, then it’s worth noting a few points here. For one, King Gwayne V, who first raised the Tyrells to prominence in the Gardener court, was the third of the the Three Sage Kings, meaning that he was ruling at a time when promoting Andals and Andal culture was at the top of the Gardener priority list. Ser Alester, as not just a knight but a clearly very talented and at least locally famous knight, was an obvious choice to be raised in favor by the first Gardener king to be knighted, and established nobility might have seen Alester’s rise as a sign of the new avenue of favor. Too, the Three Sage Kings had overseen a good bit of - maybe power restructuring is the wrong turn of phrase, but certainly new power distribution in connection with the Andal Invasion; with new noble Houses rising from the marriages of Andal adventurers and reacher daughters, the social order of the Reach was certainly to some extent in flux in this era, allowing for the sort of younger Houses rising fast that we see with the Tyrells. It is also worth noting that it was only in or after the third generation of Tyrell service to the Gardeners that the office of steward was made hereditary with the family, and only on an individual level do we see significant power being wielded by the Tyrells; this was not a case, in other words, of the Tyrells getting all the goods at once.
