When we look at the past release histories for major Kobo devices, it would appear that they've been working on a bi-annual schedule aimed at hitting the pre-holiday season sales before summer and Xmas holidays.
May 2010: Kobo eReader
October 2010: Kobo Wi-Fi
May 2011: Kobo Touch
October 2011: Kobo Vox eReader
The acquisition by Rakuten last year and the need to release the JP version of KT would appear to have altered this schedule this year. I would suspect that they may originally have intended to time the KT-JP release to occur in April immediately prior to Japan's Golden Week holidays. They did however manage to get it out before Amazon's Kindle release release in Japan.
For both Japanese as well as US/European markets, having a new product out in October is a critical factor for success during the Xmas/Year-end when a large proportion of the total annual sales of consumer electoronics products are achieved.