First of all, Roll cage tubing such as BS4 T45, ROPT510, 4130 Chrome-Moly, 15CDV6 or even CDS tubes will all pretty much weigh the same as long as the diameter and wall thickness are all the same.
The way that some roll cages are lighter than others is due to using high tensile / stronger materials and using smaller dimensions on the diameter and wall thickness.
MSA regulations state that in most cases all cages should be made from 45mm outside diameter x 2.5mm wall thickness (1.750″od x 12swg) or 50mmod x 2mmw/t (2.000″od x 14swg). This is the case regardless of material used, so any thinner tubes used will not pass the scrutineer and the cage will be unusable.
There are advantages of using higher quality materials despite not being able to weight save. The ROP510 roll over protection tube for example has an increased tensile and yield strength that will stiffen the chassis structure but also contains a high bending quality that will absorb strong impacts rather the snapping and breaking under load.
In order to use a “lightweight cage”, the design must first be homologated. Test centres such as MIRA will run test software on a submitted design to calculate whether it is of sufficient standard for MSA and FIA competition. This can be a costly process that is only often found in the top end of motorsport and also within the roll cage manufacturers that will see a return by resulting sales.