As DraftAiresMum eluded to and her explaination points out it has a LOT to do with genetics.
My two are cousins and both have long, very thick manes and tails to the ground (they've pulled out hair by stepping on it), but only the older one has long forelocks. They are treated the same, but even though they're cousins they still have some genetic differences.
I've had people ask me how I got their hair so long, (they're using over the counter and even home made stuff) but the answer isn't what they hope to hear. Genetics and diet. One you have no choice about and the other just provides the max potential (but won't make it grow better than what the genetics provide for).
The only thing you can do is take care of the hair and don't damage it. I use coconut oil on their mane, because it's healthy for the hair and makes it look good (and it's absorbed, so I don't need to remove it
), but it doesn't make it grow. I start at the bottom, holding the hair above and brush it (with the oil) from bottom to top using a flexible (metal tines in a rubber base) brush. Keeps me from pulling out hair and reduces breakage.
Just keep it healthy, cut down on breakage and avoid pulling them out. You'll eventually get longer hair, but there's no "quick fix" or way around the genetics.