Snowy owls are probably expensive.
It seems likely that snowy owls are more expensive than most other types of owl. Hagrid had gotten money out of the Potters’ vault to buy Harry’s school supplies, and he bought Harry’s pet for him as a birthday present.
“Just yer wand left – oh yeah, an’ I still haven’t got yeh a birthday present.’
Harry felt himself go red.
‘You don’t have to –’
‘I know I don’t have to. Tell yeh what, I’ll get yer animal. Not a toad, toads went outta fashion years ago, yeh’d be laughed at – an’ I don’ like cats, they make me sneeze. I’ll get yer an owl. All the kids want owls, they’re dead useful, carry yer post an’ everythin’.”
- Harry Potter and the Philosopher's Stone, Chapter 5 (Diagon Alley)
The Potters were rich, so their vault had a lot of money, and Hagrid would have wanted Harry to have a particularly special birthday present since he knew Harry had not been treated well and this was likely to be the first present he ever received.
“Twenty minutes later, they left Eeylops Owl Emporium, which had been dark and full of rustling and flickering, jewel-bright eyes. Harry now carried a large cage which held a beautiful snowy owl, fast asleep with her head under her wing. He couldn’t stop stammering his thanks, sounding just like Professor Quirrell.
‘Don’ mention it,’ said Hagrid gruffly. ‘Don’ expect you’ve had a lotta presents from them Dursleys.”
- Harry Potter and the Philosopher's Stone, Chapter 5 Diagon Alley)
Hagrid would have had the money (from the Potters’ vault) and the desire to get Harry a particularly special birthday present. It seems likely that the reason snowy owls are rare at Hogwarts is because they are expensive, and most parents do not choose such expensive pets for their children attending Hogwarts.