A few reasons. One, many people will be die-hard for the original and not accept change. In their mind, that is the best thing on the market and will never seriously look at the new version.
I know you stated “improved”, but too often IMPROVED refers to the profit margin. Manufactures will find a way to build a specific product cheaper in a manor that the general public won’t realize. An example of this is fishing reels. You bought a pair of Reel X in 2006 and loved them. 2009 rolls around and you buy 2 more Reel Xs because how much you loved the originals. 6 months of use later you realize that they weren’t the same quality as the originals – WTH They found that a loyal customer would spend the same amount of money, and they could produce the reel for $8.00 cheaper by using a cheaper bearing and more nylon parts. Not bad improvement to profit $8.00 X how many thousand reels.
Marketing for the “one-up” feature. They can charge more for the newer version and have a lessor product at a cheaper price point. Either way, they capture the sale and have a revenue stream…