Because it's no different to the support lots of other people get already for various things in society. We all fund lots of things.
In this particular case, a mortgage crunch could effect millions, a big enough number that without support could push the country into recession, costing jobs and businesses. Mass repossession would also increase the burden on the state to provide alternative housing which currently doesn't have the capacity to support a problem on this scale.
The support wouldn't extend to capital repayment or the original accepted interest, it would only apply to the interest element above a certain threshold where the mortgage holder couldn't have foreseen or have been expected to take that scale of risk.
Renters should also be protected so no increase in rents.
There is a difference in my mind to an individual winning or losing based on their own circumstances or choices, and millions of people potentially facing financial ruin through nothing they could have done about it. Yes, clearly the Government is there to protect that scale of a problem, as it did with furlough for that group, because when/if the problem reaches that level it will effect more than just mortgage holders.
Of course, this problem were facing now is partly a result of all the cash given out during COVID. The government helped make the problem, they should therefore support those now effected by it.
Obviously we're not going to agree on it, but in time a sufficient weight of people effected might force the government's hand on it.