Programmer productivity and time to market.

They are making computers faster and cheaper.
They are not making programmers faster or cheaper.

A modern fast web server can be bought for the price of 10 hours of development. Unless it is an extremely high-volume situation, it is way more cost-effective to buy more machines than to force programmers to use a less productive environment. Studies show Perl is not just percentages but factors more productive than Java or C#.

You can always use mod_perl if you don't all those instances.