This is because the Abstract Equality Comparison Algorithm requires that if Type(x) or Type(y) is a Boolean in the expression x == y then the Boolean value should be coerced to a number via ToNumber, which converts true to 1 and false to +0.
This means that any comparison of true == something or something == true results in 1 == something or something == 1 (replacing true and 1 with false and +0 for false).
The Null type does not compare as equal to either 1 or +0 (in fact, null is only comparable to undefined in the Abstract Equality Comparison Algorithm).
There is a detailed discussion of all of the different kinds of equality in JavaScript on MDN that is well worth looking at if you want to know more.
However, if you coerce null to a number it is coerced to +0 so +null == false actually returns true.