Because one requires only a theoretical breakthrough and the other requires engineering. Ideas iterate very quickly. Hardware has to be built. The machines that make the machines you want to use have to be designed, whole industries may have to be invented. A theoretical breakthrough doesn't have the same lag time.

If I work as a theorist and I have a brilliant insight, I start writing the paper tomorrow. If I work as an experimentalist and I have a brilliant insight, I start writing the grant to purchase the new equipment I'll need.