Because the two are interconnected and aren't in conflict with each other. You not only want high throughput - that by itself would be quite limiting. You want

Because the two are interconnected and aren't in conflict with each other. You not only want high throughput - that by itself would be quite limiting. You want it along with low latency as well, or else it's very easy to end up in the situation where your throughput is effectively zero if the access pattern is "bad".