From C++ Templates The Complete Guide:
Member function templates cannot be declared virtual. This constraint
is imposed because the usual implementation of the virtual function
call mechanism uses a fixed-size table with one entry per virtual
function. However, the number of instantiations of a member function
template is not fixed until the entire program has been translated.
Hence, supporting virtual member function templates would require
support for a whole new kind of mechanism in C++ compilers and
linkers. In contrast, the ordinary members of class templates can be
virtual because their number is fixed when a class is instantiated