>>951230Two reasons, really
Weight - 10cm of stem is usually lighter then 10cm of frame.
Geometry - keeps the wheelbase more compact, which makes the bike turn on a slightly lesser radius
But it is a matter of trend, really. 70ties bikes quill stems were 70-90mm. It's the introduction of threadless headset and bulky carbon stems, along with compact geometry, that allowed stems to grow significantly.
That being said, my road bike has 50mm stem and I love it.
