When using floating point numbers, there is are lots of problem with rounding and equality.
From the point of view of the normalizing committee, you need to ensure that two programs execute the same on several compilers. Therefore you need to specify very precisely what is the result of floating point operation. Probably they felt that the IEEE-754 norm is not precise enough...
So this is not a question of whether or not it is implementable but more of what precise behavior we want to have.
Note however that constexpr accept floating point values. This is usually enough for compile time computation.
