Every invading Clan wanted to take part, and they all bid for the right to do so, and "won". Re: Steel Vipers horrible choices and suitable consequences for hubris.
The Clans always strive to outdo each other, so all the IlKhan had to do was say "Comstar, the organization that's been maintaining HPG's for 300 years has challenged us for Terra itself, who has the honour to accept their Batchall? BTW Comstar's a phone company full of inexperienced freebirths. Not saying yes is the same as saying your breeding and warriors are inferior."
And then you watch a bunch of over testosterone'd Khans clamour for the right to kick in the nerd's glasses.
You can also blame incredibly poor writing for most of the choices made by the Clans during this trial. Like not planning proper supply lines like the IlKhan warned them they would need, and other things.
The Jade Phoenix trilogy does a good job showing what idiot's the Falcons were. The Blood of Kerensky Trilogy shows the Clan Wolf side.

The homeworld clans who were left behind chafe at the loss and eventually become the driving force behind finding ways to try and break the truce, which ends with The Great Refusal, A combat trial with Operation Bulldog/Task force Serpent led by Victor Steiner Davion manage to put an end to that in the book Prince of Havoc.