The important thing to remember here is that the archangels don't want this to be easy.
Although none of this has been officially confirmed as canon, I suspect that there were three main factors at play.
First off, poor information. It's not entirely clear how the archangels learn about the goings-on of Hell, but it's probably not a very good source; for all their power, the archangels aren't omniscient. It's conceivable that they don't know when exactly Dean breaks the seal, only that Alistair is working on him, so the archangels want the siege to take as long as possible to give Dean as much time as possible.
Secondly, there's an element of psychological warfare against the demons. Remember that the archangels need the forces of Hell to be running around breaking seals. They need Lilith herself to break the last one, and it wouldn't look good for their image if they were running around summoning Samhain and killing Reapers themselves. An archangels descending on Hell and decimating the local demon population would have been a major confidence blow to Hell's Army, and we see from the Crowley/Abaddon thing that demons aren't particularly loyal. The archangels want the demons to believe they have a shot at raising Lucifer, so that the demons will actually go out and try to raise Lucifer.
Finally, and most importantly, there's some psychological manipulation of Dean going on as well. The archangels really need Dean to become Michael's vessel; the whole plan falls apart without it (they don't know about Adam at this point). In order for that to happen, they need for Dean to form a positive opinion on the angels - he needs to accept that they need his help, and that this is the only way.
I believe they do that by introducing an angelic "handler", in the form of Castiel. Cas seems to follow Dean around a lot, and at one point comments that his orders are basically to do whatever Dean says. I suspect that Cas is meant to make Dean sympathetic to the cause (although he obviously doesn't know this), by emphatically and honestly describing all the ways the War is going poorly for Heaven. One way to do that? Mention that getting Dean out of Hell was a major blow to Heaven; Dean feels like he owes them one for springing him, and also feels bad that they suffered such heavy losses on his behalf.
Obviously this plan backfires horrifically for the archangels, but they couldn't have known that.
