Greed
...VAST (Video Ad Serving Template) proved insufficient for advertisers, who demanded an extension to execute code in advertisements.
At which point they should have been told exactly where to go, taking their crap with them.
I personally have no issue with "subtle" ads on web pages, as in ones that are static, small (both in dimension and file size) and simple. What I object to is having both my bandwidth and eyeballs hijacked by people who think adding a movie or interactive box to a webpage is going to either enhance the page or their chances of me actually being interested in the product being spammed at me.
Indeed in almost all cases, it has exactly the opposite effect of putting me entirely off the product and ending up negative and evasive toward it. And that's just the legitimate advertisers, let alone the malware scum that the door they opened up lets in also.
And people wonder why ad-blockers are doing such good business? With all this crap there's an argument that they're to be included in the anti-virus/anti-malware toolbox.