They have less impact on the damping factor of the amp circuit they're connected to so there's better control while sensitivity is usually helped along that barring anything powered by a battery (ie power output drops more sharply as the load impedance increases) it usually compensates for the power drop.
1. It's not such a big concern now that you have so many amplifiers with 2ohm or even lower output impedance even at lower price points.
2. Something like the HiFiMan HE400S can have 24ohm nominal impedance and yet maintain 96dB/1mW sensitivity comparable to a 250ohm Beyerdynamic while maintaining a response graph roughly comparable to a 93dB/1mW 62ohm AKG dynamic driver and a 97dB/1mW 300ohm Sennheiser, all of which are designs from over a decade ago.