They make the most sense for large installations because you can increase the capacity largely by just increasing the size of the tanks storing the anode/cathod

They make the most sense for large installations because you can increase the capacity largely by just increasing the size of the tanks storing the anode/cathode liquids. The rest of the system (pumps, membrane, etc.) stays the same for the most part and can be thought of as a fixed cost that you must pay for regardless of the tank size. So, you can potentially make a really long lasting battery with a low cost/kWh by scaling the tanks so they store a lot of anode/cathode liquid. If you try to make a really small version of a flow battery, the cost of the pumps and membranes and general system stuff will probably dominate and give you a high cost/kWh.