I have a trip booked next month and also wanted the Casitas. It was also not an option in the online check-in. I had a chat session with a member who directed me to online Tech Support. Tech Support told me that there were not any standard rooms in the Casitas. I kinda doubted this so I called reservations. It was a difficult call as the member had English as a second language, but in the end the same information. They don't have standard rooms in the Casitas. Out of 5 buildings, I find that very unexpected.
Not only unexpected, but also not true. As is often the issue with CM's over the phone, this is incorrect information. Casitas 4 & 5 are both standard buildings. Only 1-3 are considered preferred. Sadly, DISers tend to have more correct information than the CMs do.