This is probably obvious - potting on question.

the soil in the ground is alive, and constantly under renewal. When rainwater falls its highly oxygenated.
In pots things like tap water contents can kill the top few mm of microbes. The water is less oxygenenated. There is less nutrients, and no renewal. Leading to collapse if no repot or added nutrients.
Plants are highly varied things, some like restricted space, different types of soils, water, nutrients, etc etc, and some dont.
Plants are living things, they have an awareness of their environment. Its a safe bet they know theyre sat in a pot, and react accordingly. Aspects of Plant Intelligence is a good read by Anthony Trewavas.

If you have a big enough pot its worth sticking it up on bricks, digging up some earthworms, and letting them roam around inside your pot until theyre unhappy and leave.
Dont drown the pot in water though, or your friends will die. The key to a healthy plant is healthy soil and microbial activity.

That's not why you pot on gradually.
You pot on gradually because compost does not hold on to nutrients well, and the nutrients that were in there will all have been washed out after about 6 weeks, and reduced sooner still. So you move the plant on to fresh compost as and when it needs the extra space, rather that putting it in a big pot immediately and having it run out of food.

Thanks both, informative replies.