Cost. That's what business boils down to at the end of the day. I am a mechanical engineering designer for a company where everything has to be designed on a shoe string budget. I will try to break down where the savings are:
Supply.
In terms of construction steels, the most common types are RSJ (I beams), RSA (angle iron, L Beams), RHS (Rectangular and square Hollow Box Section). The huge tubular steel pipes that coaster manufacturers seems to like using the most are very uncommon in terms of construction. They are used, but usually as ties rather than struts. Anyway, this means that RHS is much more readily available in a much wider array of sizes for a lesser cost because it's a more competetive market. Above this, however, is that Box Section steel is much easier to produce than thick wall tubular steel, which means that as a standard, it is less expensive to the secondary market.
Design and Manufacture
In terms of the manufacture of the supports, firstly handling and storage of Box Section is a lot easier than tubular steel is as it can be stacked with greater ease and can be stored on normal pallets etc. Cutting of Box section is much more easily accomplished. The angular welded joints between two sections of tubular steel are quite complicated to cut into the end of the steel (I probably need a sketch or 3D model to explain this) in order to create the joint between the two sections. One straight cut on an angle is all that is needed to complete the same task with Box Section. Other manufacturing reasons apply, but I won't go into it, I'll bore you more than I already have.
Transport and Logistics
Bescause of the Box Section's ability to stack without much aid from straps and other such things, it's not only safer to stack the pallets, but it also provides you with a flatter packed pallet, meaning you can stack more pallets onto a single lorry. If it takes 100 lorries worth to get all of the steel to site, it might save you 2 or 3 truck loads. Which is a saving of a few grand straight away.
There are some other things that help acheive a saving but... yeah.
Disadvantages:
The cross section of Box Section is actually not as good at dissipating load as tubular steel. This may mean that more supports along a section of track are required in order to dissipate the forces correctly through the structure. However, if the saving on the production of individual supports is great enough, despite more supports being required, an overall saving is evidentally still attainable.
Hope I possibly cleared it up, although I may have confused you further