In the Dragon Ball Heroes videogame animated scenes, Trunks is able to turn super saiyan rage. But in the anime he doesnt transform into it, not even to fight Oozaru Cumber, he only turns into super saiyan. Now I havent read the Dragon Ball Heroes mangas, but if the manga and the anime are based in the videogame, why doesnt Trunks turn into super saiyan rage in the anime?
The only possible answer for this question is the writers decided he would use his Super Saiyan form.
If you rewatch the Episode, Goku and Vegeta also turn Super Saiyan against him at the start despite witnessing his power and fighting him fused before. Plus, they weren't out of stamina or anything as Goku transformed to SSJB when Cumber tried to crush him and he follows it with Kaioken times 20. The transformation does exist within Dragon Ball Heroes. Hence, there is no other specific answer or logical reasoning why.