Currently, you are rewarded for being faithful to your campaign at its end. Not that you run away and just join the campaign where your alliance dominates, so you get to kill many enemies and even get easy extra AP because your alliance holds many keeps...*
Does it work? Of course not. Almost every campaign is dominated by one of the three alliances and more or less dead regarding true alliance competition.**
Would it be a solution to prohibit again the same account's characters (of divergent alliances) to play on the same campaign? Maybe, but I do not think this is a way that can be pursued. We dont live in the past, and there are always ways around prohibitions...

So here's my proposal how to fix campaign balance: Provide incentives for fighting as a minority! Go where troops are needed!

1. Allow free choice of campaign every moment. Remove campaign assignments of characters. If you change your campaign, you even keep the rankings on every campaign. So you can gather multiple rewards, well, if you are a playing a lot.
2. Introduce a new AP modificator, depending on the current (hourly) population balance. If you are a minority, get more AP, if you are a majority, you get less. All old modificators stay valid, too.
3. For all our newbies: Display indicators of your current or potential AP buffs prominently. Your current multiplicator should be displayed like the telvar stone multiplier in IC. Your potential multiplicator (stemming from [2.] above) should be displayed besides the campaign which you can every moment freely choose (see [1.] above).

If ZOS impliments this, we maybe also finally get rid of locked queues, because ZOS can open up many more campaigns and all will be equally busy.

*dream*

* On how to make AP, see https://forums.elderscrollsonline.com/en/discussion/311138/how-to-make-ap by @Crown.
** EU Trueflame is maybe a bit more alive cause of some active roleplayers there (sorry, my dear guild). But basically, there is the same problem. One can feel so great to paint the map [insert your colour here]! Just, where are the enemies one gloriously defeated? The new huge offensive ticks (which should be kept for other reasons) make it even more rewarding flipping empty keeps en mass.