Because reactive programming is a pretty specific (though not overly specific) programming paradigm, while the manifesto talks about ideals in software design. You can achieve these ideals with or without reactive programming, and you can use reactive programming without aiming for those ideals.
As a model in which you define dependencies among different changing variables, and any change in the value of a variable – which can happen at any time, even concurrently with other variable updates – triggers the re-computation of all dependent values.
Both are nice, but they're two different things.