[James2001]

I know it was Channel 4 selling its own advertising that led to the end of the interval, so from January 1993 onwards you had an advert break before schools programmes started, and you only got 40 seconds of a clock.

I imagine the switching between TV-am and the ITV companies made it too complicated to try and insert an ad break in there, which is why you got a long interval instead.

I'm not sure the claim in the comments that TV-am played out the ITV Schools interval is true though.