The short story is they were no longer profitable. Gillette discovered they could make more money selling multi blade cartridges. Also, shaving tastes shifted after the introduction of the first cartridge systems. Carts are easier to use and require less skill. They are also faster to shave with. People like easy and fast. It's what our modern lives thrive on. DE shaving became something your father and grandfather did, and that was not en vogue any longer. So, all of those things combined led Gillette to phase DE's out.
DE razors were obviously built to last a lifetime. There isn't a lot of money in that for Gillette, or any company. Especially when you sell your product for $10 or less. Gillette needed to find a way to increase profits, and that meant selling more product. Planned obsolescence is the mantra for the economy of the late 20th and 21st century economy.