Is there any canon explanation why his book was left as a class copy?
I doubt it was left as a class copy.
The book was probably left in the cupboard by Snape as a personal reference, and found by Slughorn when he took over the class. Of course, after many years of referring to it, Snape probably knows everything in it completely from memory, and he doesn’t need to look at the book any more.
Harry and Ron only get textbooks from the cupboard because they didn’t buy copies beforehand, which is fairly unusual. When Snape was running the course, you’d know whether you were taking Potions to NEWT before you started the class (and Snape isn’t the one to make exceptions).
It sounds as if Slughorn expects them to borrow them, then return them when they have their own:
“Ah, yes, Professor McGonagall did mention… not to worry, my dear boy, not to worry at all. You can use ingredients from the store cupboard today, and I’m sure we can lend you some scales, and we’ve got a small stock of old books here, they'll do until you can write to Flourish and Blotts….”
Somehow I don’t imagine Snape being so generous. You turn up without the textbook? Tough, five points from Gryffindor. Ten for talking back. And another fifteen at the end of the class when you can’t finish the potion.
There was probably never a risk that a student would get that book in one of Snape’s classes, and that only opened up when Slughorn took over the class. (This means Harry getting the book isn’t just a plot convenience – both Harry and Ron fell through the gap between Slughorn and Snape’s NEWT requirements. Then it’s 50/50 which of them get Snape’s old book.)
I imagine that in earlier years, Snape would have been more careful, but he has a lot on his mind – the ongoing war with Voldemort, his Unbreakable Vow, and preparing a DADA course – an old notebook that he hasn’t looked at in years probably isn’t top of his mind of things to remove when sanitising his old classroom.