@kit I've often wondered about this too. And I agree with you – it's probably more about convention than anything else.

Although ... as I type this I am reminded of the time I tried to typeset a comic with upper and lower case and in the end I went back to all caps because it allowed me to space the lines a lot closer together.

With the lowercase, letters like y and g would dip into the top of the line below and it meant I had to space the lines wider which led to bigger speech bubbles.