Because the name " The Clock Tower of the Palace of Westminster in London" just doesn't grab the tourists...

... and frankly, it's too dang long. So, we have "Big Ben", which is actually the name of the bell ( which was originally called the Great Bell ) in the tower, but people have over time applied it to the clock and then the tower itself.

It was built in 1834 in a Gothic Revival style out of limestone. The designer, Augustus Pugin finished it and went mad, was institutionalized and died, saying it was the hardest thing he had ever worked on in his life.