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My experience with this has been an aversion to disturb the existing user-base (and normally within organizations with deep-seated users). A lot of sites where you'd previously seen this type of reluctance (IMDb, eBay / PayPal, Craigslist, Amazon, Google) seem to be some of the older sites on the top end of the internet spectrum. However, luckily that attitude seems to be shifting (to varying degrees from incremental fixes on Craigslist to broad changes with Google products). In some cases, there are certainly technological constraints but I've never seen anything that seems entirely insurmountable.