There's 2 issues that could come from this:
1. Players with a bad graphics card could find that there game lags when playing. It could also put more strain on the server as the particles can cause lag on the cpu/ram, it's like loading in a new world or a large file on world edit, it causes the server to lag or even crash.
2. Although it's still pixels, having "blood" could cause younger children to feel triggered by this. Most games that include blood within fighting usually are capped to a 16+ or higher, sometimes 12+. With Minecraft being rated a 7+, this would have to have a way to toggle off. And would take a while to code and wouldn't really be necessary.
Although the idea is good and I would love to see more kind of effects, It could cause many issues and is not really a priority.
Hope this helps!
~ Jack (GingerNinjaJ)