Leone used squibs first (without blood) in the suspenders scene in OuTW, and then with a lot of blood in the pup flashback of Giu la tresta. Apart from that the shoot-outs are all bloodless.
The simple answer is that in these days showing blood was the exception (before it became the rule in the 70s after The Wild Bunch). SWs were often extremely violent (often in a sadistic way), but the use of squibs was even in the 70s scarce, and generally there was less blood visible than in the US westerns. Sollima used early squibs in a few scenes in The Big Gundown (1967).Leone used squibs first (without blood) in the suspenders scene in OuTW, and then with a lot of blood in the pup flashback of Giu la tresta. Apart from that the shoot-outs are all bloodless.