Simply because a backpack is too much for a 2-3 hour hike. I've tried a Camelbak, but that hydration system is not for me.
i'm in the desert, so extra water is a good thing.
when my Cherokee got wrecked and i was commuting on my motorcycle ii'd drink up before i left work and still drain my 100oz Camelbak before i got home, which was usually only a half hour ride. but that was in the middle of summer. the Hi Viz orange Camelbak is nice for visibility on a bicycle or motorcycle, but that's not what i want when hiking.
some of the Camelbaks are very compact with either no extra space for anything besides water or various extra cargo capacities... but i like having it on my back and not flopping around (i used to use a 2qt canteen, but didn't like it as much as the Camelbaks... and i bought the smaller MULE for the times i don't need the capacity of the HAWG.
but the HAWG can take two 100oz bladders. i haven't set it up like that yet, but there are times i wish i'd had that ability.
but one of the biggest advantages is not having to deal with unscrewing a cap... and in some cases being able to drink from the hose.
for instance, on the motorcycle i always wear a full face helmet and you just can't drink from a normal bottle like that... especially not while moving. with a Camelbak i can drink while riding and stay better hydrated.
if just hiking, it's still convenient to not have to deal with a threaded cap.
i dealt with bicycle water bottles for many years before Camelbaks became readily available. those work but are still not as convenient IMHO.
that doesn't mean that it's ideal for everyone, but the smaller units are much slimmer and ighter than an average daypack and it's simple enough to keep it loaded with the incidental stuff that's sometimes nice o have while hiking; bandanna, watch cap, Buff, Shemagh, FAK, fire starter, backup knife, fishing gear, snacks, or whatever.
