Putting the ideals of teaching aside, I can speak briefly about it from a financial standpoint. When you're a private teacher, most often you're being paid by hour, doesn't matter who you're teaching. So, it's simply more cost- and time-effective to teach every student in the same or similar way. If you have a special student with different needs and feel no purpose for the job other than it paying your bills, you're obviously not going to expend the additional effort and resource simply for one 'snowflake'. It's much easier then to teach a typical student instead. In my experience, most private teachers already have lessons prepared for specific levels and preparing additional lessons and changes in lessons costs time that no one will pay for.
With a student that needs a different way of teaching you need to spend hours upon hours first working out the way that will be effective, essentially relearning the topic, then preparing the lesson and tweaking it when necessary. In comparison typical student requires only minor tweaks here and there like choosing different kinds of exercises at different times.
For a typical student, if I have a lesson prepared, it takes between maybe 10 to 20 min to prepare myself for the lesson. For a special student, I need to spend additional hours researching their learning methods, then applying these to the lesson, then tweaking the lesson - and it's often difficult at that. It's all capitalism, really, and if you count your time as resource you invest in a student, then it's more effective to invest in a typical student, since you invest less while getting exactly the same (monetary) gain. At the end of the day, teaching a special needs student can at times give you as little as a pound per hour and teachers have to eat too, you know.
Additionally, there is always a specific program that you have to teach that leaves little room for creativity really. Not only that, but very often only very specific answers are considered as viable on official exams - these basics are always created by the government and the teacher is always limited by these unless they are teaching a hobbyist.
Sure, there are teachers that see teaching as their life purpose but those are few and far between. Most are just people, with jobs they tolerate to pay bills. Most don't really care that much about their students either. Most, also, are quite lazy and try to do the easiest thing for the greatest gain, as is in human nature.