The rational is to avoid the deletes to be shifted out of the numbered registers too soon. Typically, if you intent to re-insert deleted text after a while, you put it into a named register you can remember. If you believe you will not need this piece of text anymore, you don't specify a register.
But if you are like me, every once in a while you will regret having deleted some text. If it has been just a few words, the fastest solution is to re-type it. But if it has been a whole paragraph or a code block, I'm happy to find it in
"9 somewhere. And I wouldn't want it to be kicked out of
"9 meanwhile by a couple of deleted single-words.
And why one line, not something like "40 characters" which may be a border when it starts to hurt to lose text? Because you don't count characters. If you are doing deletions inside a line, you are aware: This will soon be gone forever.
In your case you may still undo all changes until your delete, yank it and redo everything. There you have your deletion back.