Referring to skillchain, not the properties. Multistep (5+) same element burst and only fire is possible.
Fire is the only one that can go L1 > L2. It's annoying.
Visualization

But you can burst Fusion with Light spells too. It's not all fire. You just see fire as the mainstay in the animation.
You might find a "reasonable" answer in Light and Dark being opposites though.
The path to Light SC has a logical progression in the Liquefaction -> Fusion > Light path.
The Dark SC has an illogical one because even though there are 3 Darkness SCs as well, Compression doesn't slide into Gravitation as the opener.
Skillchains are also about going forwards and backwards. Openers and closers. Darkness demonstrates the closer, because you can go...
Transfixion -> Compression -> Makes Compression -> Detonation -> Makes Detonation -> Compression -> Makes Gravitation -> Distortion -> Makes Darkness
Sure you gotta put up with that weird Detonation in the middle of it, but you get all 3 Darkness SCs in this chain of chains as well.
I'm surprised you're not hung up on there being no way to chain all 3 Light SCs into a single path.
(Well I mean, I guess technically you could do Compression > Transfixion > Transfixion > Reverberation > Reverberation > Impaction > Impaction > Liquefaction > Liquefaction > Impaction > Fusion > Fragmentation > Light in a 12 step but... anyway I'm pretty sure the guys that came up with the skillchain properties were the same guys who came up with singing Time Devourer a song with elements using the Chrono Cross... It's a system of dualities)