1) They're a California biotech company. It's easier to say: why aren't you in China doing this, than to actually do it.
2) China IS NOT easy to operate in. You don't just plunk down $20m and set up shop. China is not a free market, there are a lot of costs to doing business in China, bribes to be paid, officials to deal with, local competitors to watch (intellectual property theft is the biggest threat in China for someone like Geron).
3) By the time they were to set up shop in China, staff their labs & offices, start trials (while avoiding not killing people, because lawsuits can happen in China just as easily as the US), ramp up to handle actual treatment programs ... they'd damn near be done and approved in the US, say 30 to 36 months from now. And the US is the world's largest and wealthiest market, what approval in the US means is obvious globally.