Not guilty implies "we didn't meet the burden of proof"
Innocent would mean "you clearly didn't do this and this case should have never been brought to court"
Say you were sent to trial for a murder you didn't commit. You lose your job because of this. You are found, under the current system, "not guilty". But it is not a "didn't meet burden of proof" not guilty" but a "you didn't do anything" not guilty. There is no way to distinguish between them on paper. Imagine how that would look when you try and look for a job. You would much rather they had "innocent" instead of "not guilty".