There's no need to focus on just one. However people are passionate about advocating for eating a plant based diet because it is so easy, relatively speaking. Getting fracking to stop at even just one site is a lot of work, but getting more and more people to stop eating animal products, because it's directly within their own control, is simple. Many are also passionate about it because there are other benefits that come with it - no longer needing to kill sentient beings (you probably wouldn't eat your dog, so why eat a pig who is of equal awareness and intelligence?) and as long as it's based on whole foods, eating plant based comes with a lot of health benefits compared to how most people's diet is (in the West or Western influenced - e.g. a lot of Koreans and Chinese have hugely increased their animal product consumption and started including animal products they didn't previously eat like cow milk cheese).

So again, we can work on more than one issue, but why there is strong passion behind diet is that it's a very direct action and comes with other positives.