SWhy is Yahoo looking for a new CEO? Because founder Jerry Yang, in his year and a half on the job, has proven himself incapable of meeting the company's many challenges. Here are five key moments where Yang failed to rise to the occasion:1. The all-too-sacred cows. When he first became CEO in July 2007, Yang promised that there would be "no sacred cows" and a 100-day action plan. Instead, his efforts became mired in Yahoo's famous bureaucracy; the cows were spared. While the company had layoffs last February, they were not nearly enough; next month, Yahoo is cutting another 1,500 employees. 2. The Microsoft deal. In retrospect, Yang should have taken the $45 billion offer Steve Ballmer made in January and run. With the stock trading below $11, the company is now worth a third what Microsoft would have paid. 3. The Google deal. Google and Yahoo had struck a tentative deal to have Google sell ads on some of Yahoo's search pages; because Google is more effective at selling such ads, the deal would have added an estimated $800 million a year to Yahoo's bottom line. But Microsoft executives, enraged at being spurned by Yahoo, made so much trouble for Google and Yahoo in Washington, D.C., that Google CEO Eric Schmidt chose to walk away rather than accept a deal that would set a precedent for regulating Google's monopoly on search advertising. 4. The Carl Icahn insult. Corporate raider Carl Icahn pressed hard to get Yahoo back to the negotiating table with Microsoft, then tried to unseat Yahoo's board. Yang and the board retaliated with a campaign on Yahoo's homepage which suggested Icahn was clueless about technology. Days after the diss on Yahoo.com, Yahoo's directors relented and nominated Icahn to a board seat. While he's been mostly quiet, Yang's unplanned exit is a sign that he has not forgotten the slight. 5. Jerry Yang. Since becoming CEO, Yang has been pressed for an answer on the most basic of questions: Why should he be the one to run Yahoo? His answers have boiled down to this: Because he founded the company. A Newsweek article last month proved his last chance to articulate why he should be CEO, and he botched it. "Bleeding purple," to use Yahoo's in-house jargon for being passionate about the company, is not enough of a reason. Just because it bleeds doesn't mean it leads.