When it was in Independence Hall, it was just an ancillary exhibit (didn't have it's own building at the time). So it didn't have anyone constantly guarding it. And it was mead of an alloy of relatively soft metals. As a result, not only could you touch it, but many people used pen knives to shave off a souvenir from the bottom. You can clearly see how uneven the bottom is these days. This is the real reason you can't touch it as easily today; not the guy with the hammer who it it many years later in a different building. It was moved to it's own building on July 4th in the rain in 1976 (bicentennial). But this "space ship" building was later replaced with a better building.