I write like the 4 shows here.
Can't give a "why" but in writing one just now I noticed that it was done in one continuous flow: bottom of vertical to top of vertical, angle down, crossbar.
There isn't a right-handed person available at the moment to see if they use a different flow.
Also can't tell whether I've always done it that way.
Do wonder if it may have something to do with how fast my hands start hurting any more when hand writing.
Tried writing a few open top 4s just now and it was physically uncomfortable.
_________________
"There are a thousand things that can happen when you go light a rocket engine, and only one of them is good."
Tom Mueller of SpaceX, in Air and Space, Jan. 2011