I'm pretty much a n00b at this type of thing; however, both of the hotels I checked out in SD didn't request a deposit. Upon offering, they said it wasn't necessary! How nice. Perhaps, you're staying at a fancier place than I and the rooms are more desired. Not sure.
There are lots of variables.
The hotel's ability to rent out the room at the last minute, in case of a cancellation.
The vacancy rate. If the hotel is never full anyhow, then they are not turning anyone down to accept a reservation without deposit, so there's no potential loss.
A lot of small hotels do not take credit cards. and they would have to inconvenience their clients too much, to arrange for a deposit.
The hotel owner's attitude. Be it how desperate he/she is for money, or simply liking to be casual about accepting reservations.
If the hotel doesn't quite live up to the hype shown on the website, they would definitely want a deposit, as the guest might want to cancel upon arrival.
A hotel owner who does not require a deposit, might get a lot of repeat business from clients who don't like giving deposits.
I'm sure there could be many more reasons that contribute to the decision making process, as to whether they require a deposit or not.