PHP keeps evolving. Every new major release there are new features. Most minor releases are about bug fixes.
More enthusiastic users tend to become very excited with new major releases because they are eager to benefit from newer features as soon as possible.
Also new major releases issue great signs a vitality of the language, reinforcing the belief of PHP fans they have made the right choice to invest on a language that is leading its market.
Other users get more concerned with major releases because new features often come with backwards incompatible changes, meaning that it may lead to more work and great headaches fixing code that was broken by those changes.
So the name of next major version was not that important as long as all users understand that it is a major new version with all the implications.
Those that were in favor of keeping it PHP 6 claimed that it would be the next logical number after the current major PHP version, which is 5 obviously.
Those that were in favor of PHP 7 claimed that PHP 6 was the name of a PHP branch that was killed in 2010. That was a very ambitious development that aimed to bring Unicode support as standard to all PHP text handling.
Since PHP 6 was aborted, proponents of PHP 7 did not want the next major PHP version to be confused with that branch that was killed long time in the past.
The majority of the voting developers preferred to avoid that confusion, so the next major PHP version will be PHP 7.