Complex sentence: When Leo Castelli saw both Lichtenstein's and Warhol's large comic strip-based works, he elected to show only Lichtenstein's, causing Warhol to create the "Campbell's Soup Cans" series to avoid competing with the more finished style of comics by Lichtenstein.
Split into simpler sentences: When Leo Castelli saw both Lichtenstein's and Warhol's large comic strip-based works, he elected to show only Lichtenstein's, causing Warhol to turn to create the "Campbell's Soup Cans" series. Warhol relegated himself to soup cans as a subject at the time to avoid competing with the more finished style of comics by Lichtenstein.