IMHO, it's just a case of ignorance. Most folks don't have any desire to learn anything about the why's and the how's of using bits and go to the trouble to learn how a certain bit works and they just don't consider that something else might work better.
TTs are one of the most widely available bits, they are one of the cheapest, and they are advertised as being very mild and great for use as "breaking" bits, "transition" bits, and "colt" bits. They are also advertised as snaffles even though they aren't, by any stretch of the imagination. People see the broken mouth on them and automatically think "snaffle" and then make the leap from snaffle to "mild". That's about as far as they go when it comes to reasoning behind using that bit.
And, so, it simply maintains the circle of ignorance. The teachers don't know any better; therefore, the students won't know any better and when/if they get to the point that they are teaching others, then they just continue to spread the ignorance.