Some Lightning devices use both sides to get USB3 speeds. I really have no idea why Apple have left other devices at USB2 performance. It doesn't make an awful lot of sense to me.
Because what's their incentive to do better? Most people only use the port for charging, which has no data speed, and the number of people who sync via wire is lower than ever and declining. USB2 was good enough for a long time, and it's not like it changed.
I expect Apple to do away with the lightning port eventually and just ship MagSafe (the phone kind) chargers with their phones for a completely wireless phone with no holes at all. We aren’t there yet (many people still like wired headphone solutions vs Bluetooth), but maybe in 2-3 years?