Because you generally don't need to be re-appointed. You generally do need to be re-elected.

In order to be re-elected you need to please the electorate, rather than do the 'right thing' (whatever that is.)

This can create a conflict of interest, such as judges prioritising 'being tough on crime' over the facts of cases.

So: In 18 US states, the courts are entirely captured by party politics, but in the other states there's at least thin pretence that this isn't the case? Hooray for US justice. /s

Also when you say "Federal judges are appointed" you forgot to say "By politicians". Unsurprisingly the British system doesn't let politicians (who will be judged like anyone else) pick the judges, it has independent Commissions† for this purpose, just as it does for deciding electoral boundaries.

And that's how the US went many months without a functioning Supreme Court, so that a political party could ensure their party preference overrode any other consideration, and today the Trump administration basically just dictates to its five tame Justices what the US Constitution "really" always meant which just coincidentally is whatever Trump wants it to mean today. Nice "justice" you've got there, hope your life is never on the line.

† The JAC (which picks most judges including this one) is a mix of existing judges, independent lay people and non-judge lawyers or law experts. It was told to pick people on merit, but only those with "good character" and that it should try to reflect Britain's diversity (so e.g. not replace all the old white male judges with more old white male judges). It appears to work pretty well.

Your question is like asking is it better to appoint rocket scientist from a pool of professors of rocket science, via a skill based appointment system, or via a popularity competition that the public vote on.