As far as I can see, Labour didn't set out to get more total votes across the country than they did under Corbyn, because that wouldn't necessarily have given them what they wanted, which was a lot of MPs. Starmer is a realist and he understands what some other politicians seem to struggle with - you can't change anything unless you have enough MPs to be sure you can vote all your policies through. There are 650 MPs in the House of Commons so you need well over half that, i.e. 325 minimum.
If you are more concerned with being ideologically pure, as Corbyn seemed to be, it's very nice to be able to say 'Look at all the millions who voted for me, that shows lots of people agree with me!' but if it doesn't translate into MPs the policies will never be implemented.
Campaigning costs money and it takes time and effort to organise. A big party like Labour has lots of volunteers and strategists but you want to use both volunteers and funding to maximum efficiency. It's pointless doing a lot of canvassing in seats where you know you're likely to win anyway. The places where you need to canvass are the ones where you might just be able to sway enough voters for your candidate to win in that seat. Tony Blair knew this and so does Starmer. (So does Ed Davey.) I am extremely impressed by the efficiency of Labour's efforts in this election. They have a huge number of MPs from a similar number of total votes as in the last election. Now they can actually get on and govern.
People trusted them to do this not because they think Starmer is 'a wonderful leader' but because they can see he is sensible, competent, measured and intelligent, he has the respect of his MPs and party workers (most of them, anyway) and there is good party discipline. That's what we need now as an antidote to the years of Tory incompetence, self-interest and backstabbing. Corbyn was unable to provide any of that.