It's not stated in the books -- besides, we later find out, a Basilisk -- what causes a person to become petrified. The staff is aware that students and Mrs. Norris have been petrified, but they simply don't know what's causing it. It doesn't say in any canon source I'm aware of that there are other creatures, potions, or objects that cause petrification. Before the end of the book, the only persons who know there's a Basilisk in the castle are Harry, Ron, and Hermione (who herself becomes petrified). All that aside, each petrification was completely accidental. The gaze of the Basilisk usually causes death.
Had Mrs. Norris and some or all of the students died, I suspect Dumbledore would have immediately been reminded of Myrtle, who died by "Slytherin's monster," at the hand of Tom Riddle (Dumbledore never believed in Hagrid's guilt regarding the Chamber of Secrets). I wager Dumbledore would have then put two and two together and remembered that Myrtle's death was connected to Tom Riddle, and he would have subsequently made the connection to Voldemort during Harry's era.
Anyhow, it's unclear how common petrification is. Not all hospitals stock every remedy for every medical problem; sometimes they have to order them in.
It doesn't appear that the Mandrake is a particularly unusual plant. The HP Lexicon indicates it's used in "most antidotes."