On Sat, 10 Jun 2006 00:45 +0100 (BST), Colin Rosenstiel wrote:

Isn't another reason simply the fact that the Aldgate terminating
platforms are between the Circle platforms so involve fewer conflicting
movements that at Moorgate or, when the bay existed there, at Liverpool
St?

That's quite correct. Terminating the Met at Moorgate (or Liverpool
Street) would cause more than twice as many conflicting moves as
running it through to Aldgate.