Why is the next Ashes series (2015) being played so soon after the last one (2013)?

The last one in Australia was played a year earlier so to avoid clashing with the World Cup next summer.

The next one In England would have been scheduled for 2017 but they moved it back to 2015 to avoid a clash with the world test championship playoffs scheduled to be played in England in 2017

Since there is so much cricket being played its now almost impossible to maintain a 4 year cycle with anything, ICC is doing the best it can but its like trying to cram 18 eggs into a box that will only hold a dozen.

The current gap is the normal one, it was the previous one that was played a year early because of the World Cup being hosted here this coming summer.

The Ashes is played every two years,the difference this time being the WC coming at an inconvenient time.

Over a 10 year period no extra or less cricket will be played by these nations compared to any other decade.

As Binga said, it is played every 2 years. 013-2015 two years.