Because the 2 clause BSD exists now. The 3 clause makes some things a little weird. What actions are included by 'endorse'? Exactly what is a 'contributer'?
And even the 2 clause has the documentation requirement. Different projects interpret it different ways, and many projects ignore it. But, if you have documentation (exactly what is meant by that?), you had better include therein the disclaimers of all the BSD-licensed projects you used. It can be a lot to keep track of.
There are other licenses out there that do a much better job of being liberal. MIT is the most popular, but there's also ISC, Fair, even LGPL, ...
And even the 2 clause has the documentation requirement. Different projects interpret it different ways, and many projects ignore it. But, if you have documentation (exactly what is meant by that?), you had better include therein the disclaimers of all the BSD-licensed projects you used. It can be a lot to keep track of.
There are other licenses out there that do a much better job of being liberal. MIT is the most popular, but there's also ISC, Fair, even LGPL, ...