You don't think the government could monitor a hundred churches if that's what it took. Since they are burning 4 a night, just monitoring 25 would give you a pretty good chance of catching them.
No, some of those rural counties might have 3 or 4 deputies out on patrol for the entire county at night. Those folks would be covering the normal domestic calls, equipment thefts, cattle rustling, drunk drivers, etc. It would be sheer blind luck if one of them was near an arson scene when the crime was committed.
And which churches would you watch? The first batch were white, so odds are you'd have been watching white churches when the black ones burned. They've all been Baptist, so odds are to watch Baptist churches, but the next batch might be Nazarine, or AME or CME or AME Zion.