I’ll post my full theory on this, since I’ve had it lying around for a while now.
Night Elves evolved from Trolls (Chronicles said it was over generations) through the magic of the Well of Eternity. When they had reached the ‘highest’ form possible (high intelligence, virtual immortal etc.) the Well maintained their form. After the Well of Eternity was destroyed, the Night Elves probably would have grown sick if not for the Second Well of Eternity on top of Mount Hyjal. The Second Well, being created from the First Well had the same properties of that first Well, so it maintained the Night Elves’ current form, passively spread through their bond with Nordrassil (connected to the Second Well) and Moon Wells, as well as Anchients of Eternity (WC:III).
So, when the Exiled Highborne moved out of reach of the Well of Eternity’s power, they started to deteriorate because of a lack of magical sustenance (aka, their form changed into what we know as High Elves now, but probably less healthy looking and more akin to Nightfallen of Suramar in overall appearance). In order to fix this problem they changed their environment (instead of them changing to suit the environment) by creating the Sunwell from a Phial of the First Well of Eternity.
Following the logic I used to explain the Night Elves’ unaltered form for 10K years, the Sunwell would have the same properties as the First Well of Eternity, thus stabilizing the High Elves’ current form (getting them healthy again) and then magically maintaining that form through that fount of power. (It probably doesn’t even have to be an offspring of the Well of Eternity, explaining the unaltered form of the Highborne of Eldre’thalas, as they soon had their own fount of power or because they were still close to Nordrassil, and the Elves of Shandaral in Northrend).