I mean, I can go into political theory and feudal logistics if you want.
But, Rowan really didn't have access to anything resembling a force that would actually be able to take the castle. If you'll recall from the first Rastedel quest, the belief of the leadership of the realms was that this was just some minor demons going on raids. In the story, this belief doesn't actually get broken until the Rosarian army is ambushed and destroyed. If the realms percieved this as a true "Greater Demon" war, then they'd be more inclined to march. But, two demon wars so soon is unprecedented and they werne't about to believe it just because Rowan said it was the case.
Rowan's political power is basically directly tied to "how fucked the situation is". And at the start of the game, things were mostly fine. This is because his power is entirely derivative of utility and connections and not from intuitions. The lower his utility the lower his power.
He could have called up some friends and personal connections, but even then he'd need to persuade them on a months long trek north through a famously hard to supply area. In Feudal society, the only people with access to serious military powers are governments or feudal lords. And calling levies for an extended campaign is a thing feudal lords are not inclined to do. If the demon force at Bloodmeen really had been inconsequential, then the cost of the expedition in terms of manpower and lost resources (Feudal Levies are also the civilian workforce) would have been huge.
Among the major political factions, the only ones in the entire world I could plausibly see helping him might be the Knights in the Tundra. But, we haven't really established that fully, and on a practical level getting the knights to comit to a forced march to the Rakshan wastes is unlikely. And the trip south and then back north would at least double the movement time.
So the only real question is "Why didn't Rowan try to go with a small strike force of allies or mercenaries". And that's a more fair question. Helayna, some war buddies, maybe even one of the heroes could have been persuaded to help him. But, even then his behavior isn't that out of character. Adding 4 or 5 people wouldn't make them anymore likely to win via frontal assault and might make stealth more difficult.
And Rowan, for all his fame in working as a unit, is by nature something of a loner. He likes to travel light, alone, through hostile country. He did it at Karst, at other points during the war, and does it frequently in the game.
So, once it became apparent that Duke Raeve and Rastedel weren't about to march an army to Bloodmeen because a small village got burned by some "minor demons", he probably deciding that a solo-stealth infilitration was his best bet.
TLDR: Since when can a literal peasant just call up an army to send on a massive months long march based on a personal testimonial?
