What is the most important reason for low turnout?

The electorate are more likely to turnout for more important elections, e.g. General Elections.
The 2015 General Election got a turnout of 66.1%.
Local elections get a turnout of 31.1% on average and the Police and Crime Commissioner Elections got a turnout of only 15%, perhaps because they are less important.
HOWEVER, the turnout at GEs has been gradually declining e.g. 68% in 2017 and 67% in 2019.

A vote in a large constituency has less influence than a vote in a smaller constituency, so larger constituencies may have lower turnout.
Safe seat areas have a lower turnout because voters believe that nothing will change.
Grimsby had a turnout of 53.8% in the 2010 General Election because Austin Mitchell had held the seat since 1977.
in 2010, 2/3 of MPs did not gain a majority in their constituency – allows parties to form a government with only a small share of the vote, such as in 2005, when Labour won a majority of seats in parliament with only 35% of the vote.
HOWEVER, Marginal seats don’t always have higher turnout e.g. Northampton North, which was 50/50 between the Conservatives and Labour in the 2015, had a turnout of 66.6%, which is not much higher than the national average.

Politics is seen as for elites who have little in common with normal people.
Women – 208 (32%) vs 51% of UK.
Ethnic minorities – 52 (8%) vs 13.8% of UK.
Sexual orientation – 45 openly LGBTQ
Education – 29% privately educated, 84% went to Uni.
HOWEVER, many do care e.g. 84.6% turnout at Scottish independence referendum 2014, 81% at Northern Ireland Assembly referendum 1998.
Some people didn’t vote in the 2001 election because they were content with Labour rule, leading to a 59.4% turnout.
The turnout is higher when the electorate are not happy with the status quo, such as in the 2010 General Election, which had a turnout of 65.1%.
HOWEVER, some people don’t appear to be happy with the status quo because they complain to their MP and join pressure groups.
The turnout increased in 2015 to 66.1% even though the status quo party (the Conservatives) remained in power.