HOLYROOD ELECTION - WHY NOT 2020?

UKIP Scotland is calling for a Holyrood parliamentary election in May 2020 on the following grounds:

The first Holyrood election was held in 1999, followed by 2003 thereafter 2007 and then 2011

The election which was due to be held in 2015 was postponed for 1 year until 2016 as it clashed with the General Election of 2015.

Therefore is there any reason why the 4 year cycle should not be maintained and the election following 2016 be held in 2020?

Perhaps there is a constitutional reason that is not apparent.  

Or the SNP don’t actually like/want this election!!