What reason can you give as to why a monument shaped as a mive obelisk was selected to honor Washington?

OK then... I was just in DC on a school trip and a student asked me this. I didn't have an answer, but I told them I'd check it out. I theorized that it had to do with being an over looker, but does anyone else have a better idea? This has to be in by today, because we have school tomorrow (after Memorial Day). Thanks!