Most fail because clients are lied to. They are shown the high-res print ready Photoshop glossies, and told "yes, you can have this, it will use all these assets. And lots of browser control overriding interactions". Then they try to actually build it, it is slow, uses the whole months bandwidth allowance for the mobile user "my whole 10GB gone? But I only browsed the web..", and will crumble under load.

The real problem is letting designers near the web. They were OK on print, well, a lot of printers had problems with the designers as well. Hmm. Maybe it is just the designers.