C++ did not have an ISO standard until well after it was first created. Much of its early life was fragmented, with compilers all working differently to the point of being source-incompatible at times.
The first C++ standard was C++98, with relatively minor updates in the C++03 standard. The primary goal of these standards was to take the "wild west" of C++ and bring some semblance of order to it. Once the dust settled and developers got over the changes, the C++ committee would focus on new features.
From Bjarne Stroustrup himself (some parenthesis removed for brevity), The C++ Programming Language (4th Edition), section 1.4.4 "The 2011 Standard":
The current C++, C++11, known for years as C++0x, is the work of the
members of WG21. The committee worked under increasingly onerous
self-imposed processes and procedures. These processes probably led to
a better (and more rigorous) specification, but they also limited
innovation. An initial draft standard for public review was produced
in 2009. The second ISO C++ standard was ratified by a 21-0 national
vote in August 2011.
One reason for the long gap between the two standards is that most
members of the committee (including me) were under the mistaken
impression that the ISO rules required a "waiting period" after a
standard was issued before working on new features. Consequently,
serious work on new language features did not start until 2002.
C++ was not even standardized until 1998, and the next few years were spent fixing errors in the existing (new) standard. Then the committee sat on its hands for a couple of years due to procedural issues that were likely unnecessary. It then took seven years from initial inception to a public document, with much arguing and editing along the way. Even then, it took another two years until the document was in a position to be voted on and ratified.
What this means is the addition of lambdas and other language features had to wait until the status quo was stabilized (1998/2003), then politics and procedures of the process dragged out the addition of new features far longer than it probably needed to (2011).
Another reason for the lack of urgency is that functors can do almost anything a lambda can do, they are just more verbose. The language as a whole was not missing much other than the brevity of lambdas and their ability to bind to local variables. However, even that can mostly be worked around by passing references into the functor.