Every Aspect offers a specific amount of Fragment Slots. It's visible when you hover over the Aspect. The first Aspect of the Titan only offers one Fragment Slot. The first Fragment of the Warlock offers three Fragment Slots. Why it's the way it is? The only explanation here is probably "Because Bungie decided so". Bungie already announced that they are releasing more Aspects in the future, so I assume the classes will receive more available Slots and get balanced in that regard.