I am sure a lot of it is more about winning votes before elections, Labour has said it won't raise income taxe, it could have said it won't raise taxes for lower income earners, freeing them up to raise taxes for higher income earners. They have not said anything about income tax thresholds, so a huge numnber of NHS and rail workers are about to get an inflation busting pay rise, so unless the tax thresholds are adjusted they may find they pay more tax on that upper slice of income.
Details on current tax bands are here
https://www.moneysavingexpert.com/banking/tax-rates/