The DSi XL is the latest installment in Nintendo's Handheld collection. Shortly after the DSi XL was announced it was confirmed that the new handheld was going to be released on the 5th of March 2010.
Looking at the previous versions of this console and their release dates, it shows that Nintendo have a history of releasing their handheld consoles around the end of Q1 or the start of Q2.
The reason for these post Xmas release dates is probably to counter any huge demand spikes, as Nintendo have had with previous hardware releases in the past. Releasing a new in-demand model close to Xmas can prove disastrous for the supply chain, as we have seen with the Wii console before this.