Why is it possible to buy rollcages with horizontal bars linking the strut towers, and rollcages without?
Basically comes down a combination of one, some or all of these factors.
1. What your intentions are with the car, (an actual requirement or just style points.)
2. Conforming to required tech rules for a certain competition series or legality for public road use
3. Cost of cage construction
4. Weight of cage construction
This design is more accurately described as a 'Safety Roll-Bar' where a 'Safety Cage' note, no longer called 'Roll Cage' extends foward of the drivers seating position. A safety roll bar consists of a a) main hoop b) two backstays c) one diagonal and d) one harness bar. Pre 2013 d) was not mandatory required.
As of 1 January 2013 Motorsport New Zealand (my haunt and for that matter most other countries) has made it mandatory to include a 'harness bar' in all safety roll bar construction. The roll bar must be a stand alone item, but by placing the bars anchorage points on the suspension towers and placing the harness bar close to this it becomes a sort of de-facto strut brace as long as the harness belt angle is correct.
Could of been a case of a basic bare minimum safety roll-bar without a harness bar was fitted to meet a regulatory requirement and the strut brace was not considered or was an afterthought