Because a brutal whoreson murderer said so. After which the Roman Empire pretty much turned to crap.
And but for a chance spearthrust during the Battle of Samarra, it might have turned out really differently.
Who, Constantine? It's ridiculous to think that the Empire turned Christian *because* of him. Emperors strted cults all over the place. None stuck except this one. Why? Why did Constantine have this great success and Julian didn't?
Second of all, the Empire had already turned to crap. Christianity managed to salvage what was left of it from the ruins of its own stupidity.