Sure Labour could add it, and yes 70%+ do support it, but you're ignoring the fact that tories don't need to convince >50% that electoral reform isn't needed, they only need to convince enough people to swing the vote in swing seats to keep themselves in power. That's maybe a few 100k people across the entire country, much much less than 50% of the electorate.

And more so, I feel this needs to be kept in the limelight to remind Labour that it is an item that is high on the voters want lists.

Bluntly, it doesn't matter what people who are already voting Labour want. I live in one of the safest Labour seats in the country, policies that turn a 25k majority in to a 26k majority are meaningless.

People who are already progressive leaning, if voting in a tory target seat, are likely to vote tactically against the Tories, regardless of party or manifesto (Lab, Lib, or Green).

So you're back to tory swing voters, and if they care about it. This is precisely why we need electoral reform, but it's precisely the thing that stops it.

The priority order must be

1. Get the Tories out (via winning)
2. Keep the Tories out (via electoral reform)

I think you can even position it as a continuation 0f the ideas that a lot of people's Brexit vote was based on. Parliamentary sovereignty requiring it to actually be representative of the population's opinion. A higher quality parliament for a parliament with greater responsibilities.

People criticise Labour for dropping pledges, or not committing to bringing in proportional representation, during a cost of living crisis and a media war led by the right-wing press that's constantly distracting people with inflammatory rage bait about small boats and culture wars.