Releasing paid-for content one bit at a time will be a terrible blow for the mod community. Currently you just have mods for vanillas/warlords/bts. So there are just three groups of users - those who have vanilla, those who have warlords, and those who have BTS. If you make a mod or a scenario, you can target one of these groups, and use the appropriate content as base material for your own mod.
However assume now you have 5 different small DLCs, each one is offering either a new civ or a new unit, or new terrain, whatever. Now there will be people who have all of them, or a certain combination of these DLCs or none of them. In fact there will be 32 different combinations.
Now if you are a mod maker and want to use something from a DLC, this immediately limits your target audience. So to be more inclusive, you don't use any DLC content in your mod. This however limits what you can do. Moreover this puts DLCs and mod makers in direct competition with each other.