Ubuntu 14.04 LTS has been released with Linux kernel 3.13 (development version) and not a stable version of the Linux kernel (3.14 or 3.12). Is thís because of many new features being added in Linux kernel 3.13?
You are making the assumption that odd-numbered kernel releases are unstable, which has not been true since version 2.6.x. — that’s 2003. See the History part on http://en.wikipedia.org/wiki/Linux_kernel.
The development (unstable?) versions are now distributed in the various developers’ git trees; the thing more resembling the old odd-numbered kernels would be probably the linux-next git tree.
You've been told that 3.13 is "just as stable" as 3.12 and 3.14. This is kinda-sorta true, in that the Linux kernel maintainers' policy these days is that every minor release deserves the "stable" label. Oh, and they idiosyncratically refer to the minor version number as the "major revision number". (Grade inflation, anyone?)
The thing that used to be called "stable" is now known as "long-term maintenance" (LTM), similar to the Ubuntu project's "long-term service" (LTS) designation.
2.6.32 (Dec 2009) 3.2 (Jan 2012) 3.4 (May 2012) 3.10 (Jun 2013) 3.12 (Nov 2013) 3.14 (Mar 2014) 3.18 (Dec 2014)
10.04.x -> 2.6.32 LTM 12.04.0 -> 3.2 LTM 12.04.1 -> 3.2 LTM 12.04.2 -> 3.5 12.04.3 -> 3.8 12.04.4 -> 3.11 12.04.5 -> 3.13 14.04.0 -> 3.13 14.04.1 -> 3.13 14.04.2 -> 3.16
As you can see, Ubuntu has not shipped a LTM version of the kernel inside their LTS release since 12.02.1 (released August 2012), and in fact they've even taken to shipping those historically "less stable" odd-numbered minor releases.
Because Ubuntu ships like clockwork every six months, and the Linux kernel's release schedule isn't tied to the calendar, the Ubuntu project has to choose whether to ship with the most recent LTM kernel (which might be several months old at that point) or to ship with the most recent "stable" kernel, period.
I don't know the backstory here, but my not-very-educated guess is that from the Ubuntu project's point of view, cutting-edge hardware driver support is more valuable than LTM designation. Ubuntu focuses on the desktop market, not the server market, so "does it support all my peripherals out of the box" is a more important question than "are we 100% sure it's bug-free."