Accents in movies about Ancient Rome (and the like)

In movies about Ancient Rome, Greece, etc., there seems to be the exclusive use of British accents, as in the case of Gladiator, 300, and in Troy, where Brad Pitt had to adopt a British-sounding accent. In the case of Alexander, it's a notable exception. But most of the time, when depicting Ancient Rome or any other civilization, the British accent is used almost exclusively.

Now, I was watching Quo Vadis with Robert Taylor, and one thing I noticed was that he sounded American. In fact, accents were mixed in that movie. That movie isn't the only one. In The Ten Commandments[i], [i]The Greatest Story Ever Told and Ben Hur, accents were mixed too. It seems that long ago, accents were more mixed in movies set in ancient times. Unlike today.

I wonder why this is the case. I've always thought that movies set in ancient times feature the British accent only because an Ancient Greek or Roman sound like an American would be hard to take seriously. But if we look at the older historical epic movies, American accents were used and nobody had a problem with that.

I wonder, if people were to make an ancient epic nowadays, and an American accent was used, would people take that seriously? Or are the times different now and it's more acceptable to hear an Ancient Greek or Roman sound like a posh and proper Englishman?

I? I could've have. I felt her! She was alive! NOOOOOOOOOOOO!