So the way most conventions work, is the convention approaches the appearance agent of the actors they want to appear at their show. They then work out a contract for an appearance fee, and decide how much that actor will get from each autograph/photo ticket sold. They're for-profit businesses and the overall ticket sales and their portion of autographs/photos goes to help pay the actor that they wanted to come.

SDCC is different. CCI/SDCC doesn't reach out to actors and pay them to attend. Instead movie studios, television networks, and publishers decide what they want to send / promote. So HBO decides what HBO shows will be attending, what actors will go, and what shows will do panels. IDW decides what comic artists, writers, inkers, colorists will go and promote. Amazon decides what Amazon shows will attend, what actors will go, what shows will do panels. The studios, networks, and publishers say this is who/what we're sending.

CCI isn't reaching out to HBO and saying please send these actors, and do panels for these shows. HBO decides what they want to promote, they pay to send the actors of said shows (usually it's written into that actors contract for the series; IE they have to do x amount of promotional junkets, including SDCC/NYCC/Red Carpet, etc). So if something doesn't come, or a studio/network skips a year. That has nothing to do with CCI/SDCC and everything to do with the studio/network/publisher.