They can't have a climate change debate because the candidates won't have anything to talk about except for a handful. A GOP Tax Cut Debate is a good comparison. All the candidates will agree on the broad picture of tax cuts, but only a couple will have actual proposals and know them which raises three problems.
1. They're the only concrete thing, therefore they're the only thing that will be attacked. And they can be attacked "from all sides" without the attackers needing to take their own concrete positions.
B. All the other candidates will "cheat" off of them while not actually knowing the details which will give heart attacks to their advisers.
iii. Unless it's going to be six hours long, their answers will be limited to 60/90 seconds with shorter responses. That's no time for any debate that doesn't jump from topic to topic regularly to provide the illusion of being both comprehensive on "key issues" and a debate.
Rick Perry's gaffe where he forgot which department to eliminate is often (coincidentally) forgotten for how it came about. There's always a GOP proposal to eliminate some department and it came up again somewhere during that month, I forget about which department, Ron Paul went in and topped it by saying he wanted to eliminate five, Mitt Romney of course went tacking towards the center and said maybe Education but the important thing was making everything work better, Rick Perry tried to slide in between the two ends and went with three. Then forgot one of them. (A bonus part of that clip is Perry has Paul and Romney on either side of him, Paul's going "no! five! five!" while Romney halfheartedly offers "Education?" or something)
Biden is probably the least worrying candidate to the DNC in any kind of debate, even a climate change debate, he has 40 years of experience seeing all of the policy debates throughout the years. (Him falling back into "that mode" when not delivering prepared lines is almost the best case for a Biden performance.) Jay Inslee wants one because he's going all in on it to differentiate himself. But none of the other candidates have zeroed in on it and outside of Sanders (who like Paul can focus on his principles to create a "proposal" on the fly as well as having the years of experience) and Warren (who would cram for it), I'm not sure any other candidates could actually bring anything to the table both different AND what the party wants to hear as options in a climate change focused debate. Some mush mouthed cap-and-trade type proposal from a few candidates, others just repeating the words "solar and wind" until the time expires, and someone realizing they can top everybody by just saying things like promising a 95% cut in [y] by 2050 without anyone remembering or holding them accountable.
"Economy" debates are really just a way to have a domestic policy debate while avoiding tackling "social" issues. You can write almost anything in as being relevant to the "economy" if you want to.