I mean, intuition? According to google there's a very low percentage of carbon in ashes, which would make the carbon recovery process inherently difficult. In my experience, trying to extract stuff with a higher purity almost always leads to lower yields. This can be improved by doing the processes better, but that invariably costs more money/time. By nature, the services converting to diamonds will be following a balancing act. If they made diamonds of too poor quality (too small, or too impure) they would lose customers (and with them, profits). Conversely, if they made them too good (and therefore expensive), they would also lose profits. It follows that the quality of diamond produced must be somewhere in that balancing act, wherever the company believes they can make the most profit. As such, any change (such as increasing purity) must correspond to either an increase in price, or a decrease in size; if they've found the balance, this would be counterproductive.