Possibly because it is warmer around the edge of the pot and the soil extra loose and it is wetter in the middle.

There must be a reason why it is always recommended that you place them round the edge, usually works for me.

I couldn't say why but if I have success with the cuttings they are easier to get out of the pot when they are round the edges, the roots don't tangle as much

@Lizzziebee - I gave the answer, it is warmer at the edges and that promotes root growth and hence you use heated propagators.

I always put a cutting into the middle as well as around the pot and they take too.
I think it might be due to better drainage so the cuttings dont get the chance to rot as much.

I was always taught by my mum to place the around the edge of the pot as for drainage, as the middle remains wet and rots the cutting as its small and can't absorb all the wetness hope this answers your question. I ve just taught my 7 year old grand daughter to do this with cutting from carnation cuttings last week as she like it and wanted to have her own.

Better drainage, definitely. Also, it's easier to separate them after they have rooted!

May I say thanks to all members who have answered my question.
Brian Yates
www.carnationculture@vpweb.co.uk

Thank you Brian for your link very much appreciated. :o))

If you are using terra cotta pots, definitely better aeration.