Why do Apple bloggers use Skype or Google Hangouts instead of iChat for video podcasts?
Awareness

Many people know what Skype is — not many know about iChat in comparison. As soon as someone says 'video conference' the immediate response by many is 'Skype!'
Familiarity

Many of the older video podcasts use Skype because that's what they started with. Familiarity is very important with people that make video podcasts for a living. They see no reason to switch because Skype works well for them.
Plugins

There are a number of plugins made for Skype for recording the call, etc. This means that for those that use Skype they'd have no reason to switch to iChat since their favourite plugin isn't available that works with the rest of their equipment.
Livestreaming

Many video podcasts use Google Hangouts because of its ease of use for livestreaming it as well. Google Hangouts can be livestreamed with practically no preparation or equipment and then immediately made available on YouTube after, which is perfect for many.
Number of People

Google Hangouts can hold many more people than iChat/Skype, which makes it perfect for large events.