I recall reading that the reason the landing gear had to be lowered manually was because once deployed there was no way to retract it. Reentry with the landing gear lowered would result in a loss of crew and vehicle.

Basically, if anything went wrong and it was deployed at the wrong time it was game over. In the grand scheme of things it was probably fairly prudent to reduce it to a big red button instead of something that could be operated remotely or automatically versus having the ability to remotely land the shuttle in the event all the crew was incapacitated.

For the life I of me I can't find the reference at the moment... I was on a wiki binge on the shuttle program about a week ago and just can't remember whereabouts I saw this.