Solgaleo is joined by fellow Legendary Pokemon, Lunala, along with Lillie, who is the canonical owner of Cosmog, the pre-evolved form of both Lunala and Solgaleo
Cards that combine Pokemon and trainers into one scene are almost unanimously more valuable than their counterparts in the same set, and the same rings true here
Most of all, the games already-plentiful generic support for Dragon cards via the likes of Super Rejuvenation and Dragon Ravine gave it a natural advantage over its spellcasting rival
This does reduce flexibility, but it does allow you to focus more on the deck's core strategy
Heck, theres a Kingdra ex from Shrouded Fable that does 50 damage per Water Energy attached, where its fairly simple to get six Energy on board