Related question: why does # heading require a space after the #, but
blockquote not require a space after the >?
The main reason for requiring a space after the # is that it is not uncommon to have things like #15 in text, and we don’t want these to be inadvertently treated as headers. (Also, aesthetically the space is nicer.)
The same consideration doesn’t apply to blockquotes, as a line is very unlikely to start with (say) >5 for any other reason. One might, of course, have something like:
Consider an integer x, where x> 0 and x
<15.
But requiring a space would not help with this particular problem, as the following is even more likely:
Consider an integer x, where x > 0 and x
< 15.
The case could be made for not allowing block quotes to interrupt paragraphs, but that is another issue entirely and should not be discussed in this thread.
One good reason for not requiring the space after > in block quotes is that, in ordinary email quoting, it is often omitted.