I am French so obviously not the best to discuss dialects but I would be curious to know what key reason would bring so many of them.

We have dialects in France, a few are very distinct but I would not call a dialect when someone pronounces a few things differently. I know that this is subjective, but still.

There are out course some mad places where they ("they" means, you know, they) call chocolatine a pain au chocolat (a French private joke, see https://www.legorafi.fr/2013/03/20/toulouse-il-se-fait-abatt... - in French from a leading national newspaper)

In the UK, some traditional dialects are almost different languages. It is not really like that anymore, but people do have whole swathes of vocabulary that outsiders do not understand.

I think in France you got rid of the diversity in a lot of ways by having the French Language Academy.

probably related to the policy of suppressing regional languages discussed in another thread: https://news.ycombinator.com/item?id=43735946

It seems likely that regional languages impact accents in the "primary" language, and even if that's not the mechanism, the cultural attitude of discouraging "different" dialects might have the same damping effect on accents.