Still wondering why Astairon chose to remain a mere Spawn when he could've became a True Vampire just by drinking Cazador's blood, it might not be Ascension, but it's still a step up from a Spawn at least, like a nice middle ground, you know
Cazador would have had to agree to let Astarion drink his blood for Astarion to become a true vampire. Or so Astarion believes. It's entirely possible this is something that is tied to the vampire's control over their spawn (they can't bite their master without permission), and Astarion could've forced it by just draining Cazador, but he doesn't think it would work, and the game doesn't work that way (if you ascend Astarion, let him turn you and then bite him, you won't become a true vampire for example).
So it's not so much that Astarion chose to remain a spawn instead of becoming a true vampire, it's that (he believes, possibly incorrectly) his only choices were remaining a spawn or ascending, and you convinced him not to ascend.
