People forget that you can just use C for the simple stuff. Many of these Objective-C classes are slow, verbose, buggy, and poorly documented compared to the equivalent open source C libraries. In many cases the C API is just as nice - maybe slightly less fancy, but polished from years of actual use. Why use ASIHTTPRequest or NSHTTPRequest when libcurl is mature and fast and cross-platform? What do you really gain by…
Foundation (and, by extension, NSHTTPRequest and NSURLConnection and such) is a few years older than cURL is. Foundation is really an implementation of the OpenStep API designed by Sun and NeXT. in 1993. cURL was released in 1997.
What do you lose by having a higher level of abstraction that is more flexible? Do you have raw profile data that shows that libcurl is significantly faster and performs better?
Newcomers to anything should be aware that old biases die hard and people on the internet should be taken with a grain of salt.