If you have a high WIS and lower INT (all sense and no brains) you might not find a willing wizard to apprentice to, while the clergy sees potential in you. Or you might simply feel a calling, like other would-be Clerics. You might have been drawn to a deity based on another aspect of their portfolio, but discover an interest in the arcane during your initiation. You might have been 'chosen' by the deity or even just been a temple foundling.I cannot wrap my mind around Arcane Domain Clerics. Why would a follower of Mystra (or other God of Magic) become a Cleric instead of Wizard?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.