The answer is that lightening is a general purpose next generation serial format.

This allows them to support different formats in the future, for instance, say when HDMI isn't popular.

The 30 pin connector didn't have HDMI on it at the time it was created and they have repurposed pins over the years regularly.

Here they can have a much less complicated solution-- a standardised serial bus over which they can stream any future protocol they want to support.

It's a lot more future proof.