WWE decided to put Lesnar on the live event instead of the September 25th Clash of Champions pay-per-view from Indianapolis because his contract calls for different pay for live events than for pay-per-view events, which typically feature longer main event quality matches as opposed to quick squash matches.
WWE simply asked Lesnar to work the Chicago event and he agreed. The Wrestling Observer Newsletter adds that Lesnar chose his opponent based on who he felt was the best Superstar on the roster available that night, that he felt would mean the most at the box office.
Source: Wrestling Observer Newsletter
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