To be fair, it took about as long for PHP 5 to gain traction. PHP 5.0 was released in 2004, but it wasn't until PHP 5.2's release in 2006 that PHP 5 really became viable and PHP 4's EOL in 2008 that people started pushing for the widespread adoption of PHP 5 (via the GoPHP5 movement[1]). Arguably, it wasn't until the release of PHP 5.3 in 2009 with its namespace support that really spurred people to take a PHP 5-only approach to development.
Heck, one of the largest PHP 4 projects, Drupal, still supports a version of their product on PHP4: it will drop that support this March.[1]
Heck, one of the largest PHP 4 projects, Drupal, still supports a version of their product on PHP4: it will drop that support this March.[1]
[1]: http://www.garfieldtech.com/blog/go-php-5-go
[2]: https://groups.drupal.org/node/390343