Well it has been the tradition, and from a long history of western colonial dominance it has become (generally) the worldwide norm, the same way that western dress and the English language have become dominant in governments worldwide. Some countries, (Saudi Arabia is the best example) don't use the Gregorian calendar, and have their own variants, often based on the local majority religion.

That's not to say there haven't been major attempts to change the calendar and use a secular one. After the French Revolution, as part of a massive Enlightenment effort to secularize and rationalize the state, a secular calendar was created that lasted until 1805. (Another, much more successful product of this effort was the metric system.) The calendar featured a ten day work week, and had days and months named after the natural world at that time of year. Year one in the calendar was 1792, the year the French Republic was declared. It's pretty interesting, and the wikipedia article is pretty good: http://en.wikipedia.org/wiki/French_Republican_Calendar.

The calendar was brought in mainly for, as you mentioned, a desire for a secular calendar not based on Christianity. The main problem was that the calendar was poorly designed, and unpopular. It featured a longer working week and was relatively awkward to use, so many people opposed it.

While we basically use a Christian calendar because it's what's easy and no one really wants to undertake a massive effort to change it for no major benefits, it's not outside the realm of possibility that had a more popular universal calendar been adopted in the French revolution, we might all be using a secular calendar.

In fairness, most historians now use CE and BCE instead of AD and BC. I suspect that's as much change as we're likely to see.