To be qualitative, however, the reason is that Steam's server-side infrastructure is shit and its client-side application even worse. Steam is the epitome of bad design and you should feel bad for entrusting your expectations in their lazy, deadbeat, bovine hands.
They could ease the problem just a little by optimising the way Steam sends and/or receives data (so it doesn't get bottlenecked alongside useless adverts and personal information they'll sell anyway), but no, they wouldn't do a god damn thing because they know every kiddie who uses Steam has a decent-if-not-powerful rig which can handle the extra processes.
Fuck you and your stupid gaming bullshit. Buy a Mac, cockbité.