The answer is very simple: Because the PSR-740 has 4 variations per style, so each style takes up more memory, and probably the PSR-740 has about the same amount of memory as the PSR-730, so it's not possible to add more user styles.

And when you will count, you will see that the PSR-740 has more user styles than the PSR-730, because the PSR-730 has 4 user styles and 2 variations each = 8 styles; and PSR-740 has 3 user styles and 2 veariations each = 12 styles!