Why do dentists dislike using nitrous oxide?

Maybe They Do!: It may be that some dental offices prefer, or have more experience with, local anesthesia and premedication exclusively for procedures requiring anesthesia. The use of n2o isn't necessarily the anesthetic of choice, or the one preferred, for many dental procedures.

Depends many factors: In my area very few people ask for it and the city regulations (san francisco) make it not worth the trouble. Patients do not want to pay for it out of pocket and ins will not reimburse the dentist. Today there are other options for anxious patients such as pre-medication with valium type drugs. Also now we can buffer the anesthetic for more profound anesthesia.

Not necessarily: Nitrous oxide isn't necessarily disliked by dentists, but it just adds one more procedure to monitor while doing the dentistry. Sedation dentistry is becoming more popular, having the patients take some meds prior to the appointment to relax them. Dentists have their own preference and you should get different opinions on it to decide on your own. It does cost the patient more too.