Because I prefer to use higher level constructs whenever I can. "Lambda vs mem_fn/bind" is the "raw for loop vs algorithms" of the functional constructs. :-)

Do you have the book "Effective Modern C++" by Scott Meyers?
Item 34 says "Prefer lambdas to std::bind".
The reasons are

• more readable (in general, and especially when putting the result into an auto variable)
• less of a source of error (e.g. if one argument is a function call it will be called when the lambda is called, rather than immediately at the time of bind).
• more expressive (because you can define the type of capture, while std::bind always copies its arguments - which means you can use std::ref() to "capture" by ref, but that's a hidden gem compared to the more explicit lambda capture syntax)
• in some cases, lambdas are more efficient (can be inlined at calling site)

std::bind is better only if a move capture is needed, and you can't use C++14,
or with a polymorphic functor (which can be a lambda with an auto parameter if you can use C++14).

You're free to disagree with Scott Meyers though, in a case like here where none of the above arguments actually apply (other than readability, arguably). I just wanted to relay that piece of advice which I found useful and easy to memorize ("prefer lambdas to bind" is certainly easier to "it's complicated, sometimes both work, sometimes lambdas are better") :-)