Why does the qgis LTR release date lag behind the current release?

2.14 was initially released as a normal release and will be released as LTR when 2.16 (End of June 2016) is released.

This was introduced as a cool down period. This helps to identify and fix bugs (which inevitably are introduced throughout a development cycle) before releasing a version as "LTR - ready to be deployed to your critical production infrastructure".

As such, the current soon-to-be-LTR can - and should - be used to evaluate, test and harden the migration and application.

I think it is mentioned in the changelog for QGIS 2.14.0:

If you are upgrading from QGIS 2.8 (our previous LTR version) you will find a great many new features in this release. We encourage you to peruse the changelogs for the intermediate non LTR 2.10 and 2.12 releases as this QGIS 2.14 includes all features published in those releases too. Note that 2.14 first enters the regular package repositories and will not immediately replace 2.8 in the LTR package repositories. That will happen when 2.16 is released.