Why is the restrict keyword not part of C++?

There are several issues in defining "restrict" in C++, some of them are listed in WG paper N3635: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3635.pdf "Towards restrict-like semantics for C++"

Some possible issues with restrict in C++ are:

• Restrict Class members and indirection with “this pointer”
• Passing of restrict qualifiers into functions, functors, lambdas, and templates
• Escaping of restrict pointer values inside functions
• Overlapping array members, strides

Document also list several C++ compilers with limited "restrict" support for C++.

There is also interesting history note in N3635 about non-inclusion of restrict to C++:

At the time of reviewing C99 feature inclusion in C++ during the Mont Tremblant meeting, restrict was considered but was waiting a paper proposal although none came forward....

Restrict is a C99 feature and was never designed to work in class abstractions and it may have to do with that pointers are not common in C++. ... it was designed for fine-grain aliasing for C, but not well-designed for type-based aliasing in C++

Not to detract from osgx's answer, but - there is a somewhat more up-to-date paper, N3998 by Finkel, Tong, Carrouth, Nelson Vandevoode and Wong, from May 2014:

Towards restrict-like aliasing semantics for C++

And an ever newer one from 2018:

[[assert: std::disjoint(A,nA, B,nB)]]: Contract assertions as an alternate spelling of ‘restrict’

(Thanks @MCCCS for pointing the last one out.)