Devil_Hex
What if people with kekkei Genkai have an equilibrium of yin and yang in their body(like a little where they combine the elements to produce the Kekkei Genkai) which in turn lets them combine their respective elements to create their kekkei genkai elements.
This is the only reason I can think of, maybe it needs Yang release or yin release added to the elements or maybe a certain proportion of chakra is needed, take for example "Tailed Beast Ball is made from a proportion of 8: 2 black positive chakra (+の黒チャクラ, purasu no kuro chakura) and negative white chakra (-の白チャクラ, mainasu no shiro chakura).[2] it into a sphere (or a conical shape in the case of the Ten-Tails) and then fire it at the target" perhaps the other elemental releases need a different proportion for each and the Kekkei Genkai help to do this instinctively.