Would you not think it sends a message that no one is above the law, not even the police, and no matter what the time scale is someone has to pay for their errors that people died going to a football match and never came home.
I think it sends a message that the police are an easier target.
The police didn't create the problems. They did fail to find a solution, but they didn't create the problems.
1) The stadium did not have a safety license and hadn't had for several years. So why was the match allocated there at all? Why aren't the club and the FA being prosecuted for gross negligence manslaughter - they should have known that it was unsafe to have the match there. The club especially, since they were at the stadium very frequently and thus in the best position to know it was unsafe, but also the FA because they should have known it hadn't had a safety certificate for years.
2) The pen had an actual capacity of ~1600 and an official capacity of ~2200. That's negligent in itself, since the official capacity should have been changed, but the problem was much worse because ~3000 people were in there. How did they all get tickets for that? Did they all have tickets? FA and club again and particularly the club again if all the people did have tickets.
3) It was already known that the entire penning system was unsafe - this wasn't the first time people had been crushed in a pen. It wasn't even the first time at that stadium. FA for this problem.
4) The kickoff wasn't delayed even though the police asked for it to be so they had more time to deal with the dangerous overcrowding
outside the club. Is that the club's fault or the FA's fault? It's certainly not the police's fault.
5) The whole approach to the stands from outside was far too small. Could be the club's fault. Could be the local council's fault if planning permission was sought and denied. Could be hooligans' fault because the requirement to keep the opposing teams' supporters apart was a big factor. But not the police's fault.
The main mistake the police made in crowd control was opening the main exit from those stands as an entrance in order to stop people being crushed
outside the stadium, assuming that there was enough space inside the stadium for all the people who supposedly had tickets for that area of the stadium. If they hadn't, people would probably have been killed outside the stadium and we'd be in the same position we are now.
I think the club and possibly the FA were more negligent than the police, but the police are an easier target. The club is now legally a different entity and thus can't be prosecuted.