Hi,
I can understand why one can be annoyed by PHP requirements.
But even the oldstable Debian version (released in April 2015) shipped with PHP 5.6.
The old Piwik version (2.x) required at least 5.3.3.
Supporting old PHP requires a lot of afford as every new feature needs to be tested or are even impossible (e.g. for securely storing password hashes one needs at least PHP 5.5.
And as only a small portion of Piwik users are using such old versions and the time of the piwik team is limited, the time is probably better spent fixing bugs and adding features that are used by more people.