Written by Dyami Millarson
The reason why the blood sacrifices of Germanic religion tended to be communal instead of communal is practical: it was no easy task to slaughter large animals such as cows and horses (see more examples here and here). An individual could slaughter a cock or pig on his own, but a cow or horse is a whole different story.
Naturally, small animals would have been more closely associated with small feasts, large animals with large feasts. The latter would have been a lot less frequent than the former. After all, one does not need to slaughter large animals that frequent as their meat can feed human beings for a longer period of time if preserved properly.
Another reason for why the Germanic peoples tended to perform blood sacrifices together is cultural: people in the Middle and Classical Ages had a strong sense of community and they tended to do everything together. One might get a sense of this in tight-knit traditional village communities around the world.
People living in the medieval and classical times were a whole lot more communal than people are today, and so it is to be expected that it was only natural for them to perform slaughtering rituals together.
When it came to slaughtering large animals, obviously more knowledge and skill was required than with slaughtering a small animal. Here is where trained professionals might have come into play; people who served as priests must have possessed more intimate knowledge how to perform the slaughter than others. Priests are religious leaders and as such, usually distinguished or prominent men such as (petty) kings and earls took the role of priest in Germanic society.
So, to some extent, the slaughter of large animals may have been outsourced to experts as we do today in the modern world. Smaller animals could, of course, be slaughtered by anyone with some basic skills, but large animals obviously required more skills and so it was convenient to let skilled priests take care of it.
Priests combined the slaughtering ritual with fate-reading and other religious elements, so the slaughtering itself had higher significance than merely just slaughtering an animal for food and absorbing the power of its spirit. The priests could discern fateful messages during the slaughtering process and they would have looked for signs that the Gods accepted their bloody offering.
Blood sacrifice is nothing more than the highly ritualised form of slaughtering animals for meat; it has a close relationship with food and eating. The knowledge for blood sacrifice was essential for life and it was inherited. The knowledge for slaughtering small animals would have been common among all folks, but that of large animals would in all likelihood have been reserved to the most knowledgeable. So, priests would have had the function to transmit knowledge of how to slaughter animals properly, and this knowledge, as was all knowledge in the Germanic pagan world, was closely linked with the traditional religion.