I think the reasons probably variable. I think it can be true of women too, BTW.
Sometimes, it will be because the person (male or female) has matured and learned about their strengths and weaknesses, so they do a better job as a partner and parent. They may also have chosen a partner better suited to them, so fewer conflicts/problems within their relationship- I think this leads to a more harmonious home life, which is reflected in the wider family dynamic/relationship with children etc.
They may be a less anxious/distant etc parent because they feel more confident- perhaps again due to being more mature/knowing where they went wrong before, or because they have a partner with whom they work better as a team.
I know that as I have progressed in my career, I have more leeway/control over my hours/working patterns and so can be around to do more than when my children were younger. I think if I had young children now, in some ways, I would be a far more relaxed, physically present and less harried parent.
Additionally, as we are financially more secure, we can afford to outsource some things, giving us time we would have spent doing those things with our family or our own hobbies etc. I think this definitely makes a difference to our relationship with each other, and changes the atmosphere in our home. That won’t apply to every couple, but these are examples of why things are different at different times in our lives.
I think there is a tendency towards an assumption on MN that men are nearly always the reason a relationship fails/at fault (whether they instigate the split or not), which I don’t think is accurate. I suspect lots of relationships/marriages end because the 2 partners were not entirely well suited or able to meet each others needs from the start, which may come to a head over one partners actions/lack thereof, but was never just down to one partners failings. I’m not saying men are never solely at fault, just as women can be solely at fault. I just think a lot of the time, both partners did not chose someone who is the best choice for them the first time round, but do learn from their mistakes and chose a more suitable long-term partner second time round.