Comment 128 for bug 1522422

PHP7 is incompatible with many PECL extensions which are used by a LOT of websites, and are therefore VERY popular.

Any release of Ubuntu which doesn't include PHP5.6 out of the box, is going to be a non-starter in the server world. PHP7 is just too new. No one has had time to adapt all of the incredibly important and vast collection of PECL extensions to PHP7 yet. PHP7 will take years to adopt. Pushing out an Ubuntu release which is PHP7-centric is most defintely a non-starter.