[schoen]

These are great guesses, but it doesn't look like they match with what's known about the earlier forms of these letters.

https://en.wikipedia.org/wiki/History_of_the_Latin_script#Ul...

Particularly, the degree of sharpness or roundness of a particular letter may have changed a lot over time. I guess it's still possible that there's been some kind of psychological effect at certain times, but it doesn't look like it was a strong and consistent factor in the history of the alphabet.

[rusticpenn]

That is because of the focus on writing itself. Its easier to make sharper letters on clay tablets, making such letters on palm leaves ( popularly used by some cultures for writing) would tear the leaf, so the writing became more rounded in these regions.