Usually, Ubuntu tried to use the latest bleeding-edge technology. One of them is kernel. Kernel is the core of the system as it's the main application that controls all aspects of the system itself. Without kernel (and applications), there won't be any working system.
I'm wondering, why wouldn't Canonical shippped Ubuntu 9.04 with 2.6.29 and they were going out with 2.6.28 even though 2.6.29 has been out for some time and the kernel developers has released their first update, 126.96.36.199.
Does the reason related to the default logo for 2.6.29 which used Tuz instead of the original Tux? Because Ubuntu will last for about 18 months, meaning that since April 2009 until October 2010, it will show Tuz instead of Tux and people who used Ubuntu will not see other than Tuz until 9.10 is released or they compiled their own kernel.
IMHO, the time frame for testing 2.6.29 is enough to include it on 9.04 release schedule, but in the end, it's up to the developers