From an in-game perspective, why should the local rulers care about the orcs? It's a difficult trip - either through a dungeon, or overland through tangled forested hills and low mountains to get to the orc's ravine/valley camp. It'll be costly in men, treasure, and time. And your opponent are a large group of orcs who aren't bothering Stericksburg in any significant way and who are a potential, but not actual, military threat only if you provoke them. Skirmishes happened here and there in the years past, but nothing major and mostly with people who went up to Felltower or too far into the unclaimed woods to the north. And if you win . . . you get a place that's difficult to hold and doesn't come with much value. So why organize an army?
From an in-game PC perspective, why would you want an army to help? If they come and help and leave, they've done nothing but cut your treasure share to very little. If they come and help and stay, they've done that plus Felltower is now surrounded by the ruler's controlled lands. That ruler might extend his rule into Felltower, at least in name, and thus legitimately ask for taxes or tithes of loot and claim particular things . . . or require tasks by social and legal force. Why get them involved?
From an out-of-game perspective, why have NPCs do stuff for the PCs when the PCs find it hard?
That last one is why I set it up so the first two paragraphs are the situation. I made it a problem no army is going to come solve so the PCs have to do it.
This isn't a new thing - neither the orcs, nor the idea that NPCs should swoop in and make life easier for the PCs, or give the PCs a big reward or a lot of help to do the things the PCs find important.
I posted about this back in 2014.
So the PCs are the ones who need to do jobs.
But NPCs will tell them what to do and how to do it and reward them for seeking treasure, right?
The idea of the game is to be a limited sandbox. The PCs will get stuff they can do, but what to do - and doing it - is PC-driven.
I've occasionally offered rewards for items pulled from the depths, but they've almost never been taken. I've offered extra rewards for doing particular tasks, and mostly the response has been to bad-mouth the NPC offering the reward as being too cheap or the task too hard. So even the quest-lite things I've offered have been shot down . . . and honestly it's for the better. The PCs should determine what they want and how to get it, not wait around for NPCs to tell them what the NPCs want and bribe the PCs to get it. Nevermind the PCs will expect to keep the quest findings if they're better than the reward and claim they can and should be able to hold them against all comers, because that's only fair (cough, cough, White Plume Mountain, every time.) So why bother? I just make it purely about the PCs. And I structure the game in the game world to make it so.