Short version:
The Clans underestimated the IS forces they would be facing(1), the invading Clans were seriously outnumbered, Clan "Military" "Tactics" were poorly suited for fighting an actual war(2), the IS had better Aerospace pilots(3), and last but not least, the IS had the logistical advantage, as stated above(4).
Long version:
1: The Clans more or less ignored what little intel they had on the IS. They went in expecting small numbers of under-equipped, unskilled enemies, and they can came face-to-face with scores of battle-hardened, often fanatical enemies with access to almost Star League-level Tech. Say what you will about the Succession Wars, they did a good job of honing the armies of the Inner Sphere.
2: I'll take a lot of flak from the Clan Fanboys for saying this, but it's a fact. The Clan idea of "War" is essentially a glorified gladiatorial match with things wagered. Likewise, Clan troops are glorified gladiators. Don't get me wrong, Clan society produces half-decent warriors, but it produces absolutely horrible soldiers, and the "Tacticians" and "Commanders" produced are even worse. There's an old axiom; "If you're fighting fair, you aren't fighting to win." . Armed conflict is a terrible thing, and most attempts to sanitize it or salve your own guilty conscience will only drag it out and make it worse. The faster the war is over, the faster the survivors can go home. If that means using "Dirty tactics" or "Cheating", so be it.
3: See battle of Radstadt. Outnumbered and and outgunned, Rasalhagian Aerospace pilots beat the tar out of the Clan fleet, and even managed to kill the ilKhan. And before anyone says "Oh, the Rasalhagians were defeated at Radstadt!", WRONG. The Elected Prince escaped, and the Rasalhagians inflicted disproportionate casualties and damage on the Clan fleet, and the death of Leo Showers stalled the Clan advance for months. Yes, the Flying Drakons were wiped out, but that's war.
4: Wars are won by logistics. In this case, any Clan reinforcements and almost all of their supplies had to come all the way from the Clan homeworlds. In any situation, IS reinforcements and replacements were at most a few jumps away, Clan units had to wait months for their own. "But Starne, Clan hardware was clearly superior!" you say. In the strictest sense, yes. But what good is that fancy Tech if it breaks down and you have to wait for weeks or maybe even months to get it fixed? Another side to the technical issue is that not only did the IS forces have no qualms about salvaging Clan Tech, they were practically giddy at the chance, and generations of salvaging and making-do made them very, very good at it. Clanners? A whole 'nother story. Not only would the vast majority of Clan warriors be sickened by the very idea of going into battle with IS equipment, in many cases, it's not really an option. I highly doubt you could scavenge parts for that fancy "Timberwolf"(MadCat) off of a Marauder or Catapult.
Frankly, I doubt that even if all 19 Clans had fully mobilized their "Toumans", and attacked the IS all at once that they would've fared any better in the long run. The further they pushed into the Inner Sphere, the stiffer resistance would've gotten. By the time they reached Terra, they would've been facing resistance that would put religious fanatics to shame. Even worse for the Clans, if they all sent the bulk of their forces to the Inner Sphere like that, I have little doubt they would've had lots of slave revolts on their hands(You're lying to yourself if you think the "Lower Castes" of Clan Society are anything but slaves to the warrior caste) on the homeworlds.
In the end, the Clans were doomed, no matter what they did, no matter what happened. Any half-attempts at invasion of the IS(what they did in canon) were doomed to failure. Any all-out assaults on the IS were doomed to failure. And even if the Clans had stayed-put, I have little doubt that within a few centuries, the IS would've come to them, and I highly doubt that would've ended well for the Clans.
Edited by Starne, 07 July 2012 - 08:54 PM.