The Platypus reservoirs are a good example of the ultralight philosophy: high performance at the lowest weight. The Camelbak bladders have the large, heavy cap/fill assembly plus the weight of the bladder material itself, and they are more expensive. The Platypus is just a basic bag with the fill and hose connection in the same fitting--- half the weight and 30% less cost for a 2 liter assembly. It parallels pack design: stripped down minimalist examples vs heavy, feature-burdened ones.
You will also find a large proportion of UL hikers using recycled drinking water bottles and not using bladders at all, carrying them in outside pockets. Some use the Platypus bottles in the same way. This was in revolt to the polycarbonate Nalgenes which are 6.2oz and $10 each, vs the drinking water bottles that are under 2oz each and nearly free. Typically using two, a hiker can drop half a pound and spend less, which is much harder to do with a down sleeping bag or shelter, where an 8oz drop might cost you hundreds.