My guess -- the tapered towers were a proven structure with stability to handle high winds, etc. The towers don't need the underground engineering that the newer signs had in that era (for example the big Stardust sign). Put some letters on the towers and you could see them from a distance. Not as expensive as the fancier towers either.

Remember when the LV Hilton giant sign partially blew over in a high wind -- and that was built just 10 years ago or so! Those desert winds can really blow!