Why do I keep blowing head gaskets?

Wouldn't detonation show up on the plugs?
Not necessarily. In my experience the rod bearings ALWAYS show the signs of detonation. The pistons most of the time show signs, depending on material makeup, and the plugs can be hit or miss. I think there are far too many factors to have definitive answers to questions like that. AFR in the chamber, piston design, chamber design, cylinder head material, spark plug location all I think would play a role.