We assume it's because it's a fist, which is undeniably more badass than an open-hand slap (paper) or angry pointing (scissors).

我们假设这是因为当你出石头时，你的手是个拳头，不可否认拳头比伸开的手掌(步)或是愤怒的手指(剪刀)更有气势。

youdao

I couldn't help but slap the palm of my hand to my head and think, "What have I done?"

我忍不住用手掌拍了下我的脑袋，“我到底做了什么呢？”

youdao

I slapped the flag into their boss's hand – ignoring his "if you do that, I'll slap you in the face" – and began to stomp off to join our contingent on the other side of the pitch.

我把旗帜啪地一声塞到他们的教练手中，也不管那教练口中的“如果你真要这么做的话，我就要把旗帜拍到你脸上”，然后迈着沉重的步伐走回了我们队的场边。

youdao

But even while Obama holds a gift in one hand, he uses the other hand to slap business with more taxes and regulations.

FORBES