There are not the resources. There is no police service right now that has the capacity to, in a meaningful way, track a violent offender released on bail to ensure they're complying with their conditions. To do that requires a significant allocation of resources and it has a significant impact on budgets. You're allocating resources you now can't use for some other important service you're providing to the community, or you're not responding to something else because you're reallocating resources.
It does happen, on occasion, that surveillance teams are assigned to track a particular offender when there's a significant risk to the community, but there are huge implications to that.