Lisa answered this elsewhere. Under 30,000 subscribers a video show doesn’t make enough sponsor income to cover the production costs, presumably because the amount a sponsor is willing to pay is proportionate to the number of people their product is getting in front of. That means that any new video show would run at a loss until the numbers cross that threshold, which is a pretty strong disincentive to try anything new. Audio show costs are tiny by comparison, and taking that path allows them to try new ideas without a big drain on the finances, and with the option of going to video if the subscriber numbers take off. Leo’s already said he’d consider video if the subscribers go over the magic number.
Plus a lot of the wider potential podcast audience is only familiar with audio podcasts anyway, so unlike TWiT regulars they are not going to be looking for a video option. Part of the thinking for a new show is probably whether it can grow the overall audience by drawing in people who aren’t familiar with the existing TWiT approach.