Why is the triangulated category of motives easier than the abelian one?

Existence of the abelian category of mixed motives over a field makes (extremely hard and widely open) Beilinson-Soulé vanishing conjecture to be manifestly true: it follows from the fact that $$Ext_{MM}^i(\mathbb{Q}(0),\mathbb{Q}(n))$$ vanishes for $i<0$. On the other hand, this does not need to be true in the triangulated category of mixed motives.