Biden is seen as a "loser." Of all those candidates, except for Inslee's accomplishments in WA while he's been the governor, Biden is actually the one with the longest, strongest overall record of commitment to all the issues -- environmental, energy, fresh water, air, climate, ag, public health and more -- that make up the climate issue.
He started back in, what, 1972?, when almost no one was listening to the sky-is-falling climate warners and very few could get elected or reelected by concentrating on the biggest "green" issues of that time of clean air and/or water. Ever since then Biden has been one of the most reliable, and eventually important men in congress for "green" organizations.
All liberal Democrats who started in congress in the last decade have a 100% or close to "lifetime" record on climate, as rated by the League of Conservation voters, who keep records on all members of congress since I think 1971. Democrats have become aware and being for climate action is required now. Having a good lifetime record starting decades ago when most voters didn't expect or reward it is a whole different matter.
Btw, the liberal candidates in congress all have 100% annual records now. Biden's congressional record stopped back in 2007, just about the time all our caucus's high scores started accumulating and pulling up lifetime averages; so it misses out on that lift, but it's still good. And, since he's been working on them for decades, what he knows about the many technical issues involved has to outweigh by far what most others do.