From the Conservative mainfesto (p65):
continue to increase the State Pension through our triple lock, so it rises by at least 2.5 per
cent, inflation or earnings, whichever is highest
From the Labour manifesto (p48-9)
We will keep the triple-lock so that the state pension increases by inflation, earnings, or 2.5 per
cent, whichever is highest
From the Libdem manifesto (p49)
Legislate for the Liberal Democrat ‘triple lock’ of increasing the State Pension each year by the highest of earnings growth, prices growth or 2.5%.
From the UKIP manifesto (p20)
The ‘triple lock’ now guarantees the state pension will increase each year by the higher of inflation, earnings or 2.5 per cent.
There's no good reason for pensions increasing by a minimum of 2.5%. If wages and inflation are rising by less than that, it's simply a giveaway. And it's being done for one reason and that is that pensioners vote a lot more than young people. Over 65s had a turnout rate approximately 1.5 times higher than 18-24s and approximtely 1.3 times that of 25-34s (section 2.3 of this).
All that parties care about are people who vote. That's how they win. They might pay lip service to the concern of people not voting, but unless they think it's affecting a group that generally votes for them, they aren't going to care too much.