Just common sense
First time mothers aren’t always so well educated. Maybe today - at least those who have the desire and means to have a child - but a lot of them are struggling very hard to understand all the ins and out of it and are overwhelmed.
MMR and two or three other shots at the same time for a small baby/child might not seem innately abnormal to them if the ‘doctor’ tells them it’s okay. Personally, I know of two cases where it wasn’t. In reality, I’d avoid a doctor like that like the plague.