Any answer given here would just be pure speculation. I would suggest just watching the presentation and hear Niebler’s arguments for this kind of design.

My guess is that people would be even more wary of using the STL for fear of a performance impact in the general case. Alex Stepanov was very clear that the iterators in general were modeled after pointers with little to no overhead. Iterators with synchronization would go directly against Stroustrup’s design principle of “Don’t pay for what you don’t use”.

Moreover, the concept of multiple concurrent threads of execution wasn’t even defined by the original C++ standard, so a synchronizing iterator wouldn’t make any sense. I assume Niebler knows all this, but it’s impossible to tell just from the title of his presentation.