They thought all nukes inc dirty briefcase bombs were accounted for.
Then the DS showed they may have one.
Abundance of caution.
What I’m surmising is that there is a core “Plan” Q talks about, and add-on plans that would make things easier. Easy to arrest them all in one place, but it didn’t workout and we are going with the core plan. Arrest wherever that all are individually. Difficult and we may lose more operators, but less risk of collateral damage to civilians.