Why STL implementation is so unreadable? How C++ could have been improved here?

For instance why does most members in STL implementation have _M_ or _ or __ prefix? Why there is so much boilerplate code ?

What features C++ is lacking that would allow make vector (for instance) implementation clear and more concise?

Implementations use names starting with an underscore followed by an uppercase letter or two underscores to avoid conflicts with user-defined macros. Such names are reserved in C++. For example, one could define a macro called Type and then #include <vector>. If vector implementations used Type as a template parameter name, it would break. However, one is not allowed to define macros called _Type (or __type, type__ etc.). Therefore, vector can safely use such names.

Lots of STL implementations also include checking for debug builds, such as verifying that two iterators are from the same container when comparing them, and watching for iterators going out of bounds. This involves fairly complex code to track the container and validity of every iterator created, but is invaluable for finding bugs. This code is also all interwoven with the standard release code with #ifdefs - even in the STL algorithms. So it's never going to be as clear as their most basic operation. Sites like this one show the most basic functionality of STL algorithms, stating their functionality is "equivalent to" the code they show. You won't see that in your header files though.