This is either co-planar waveguide, or co-planar waveguide above ground. There appear to be filled vias in the top ground, so these almost certainly go a ground in the next layer down. CPWaG is a nice construction that allows easy transition to microstrip, to connectors, and to surface mount components.
This is a demo board for a component. Such a board may simply be measured by the customer. Or, it might be modified, or even built into a prototype demo system. As such, it has to be easy to modify and probe.
There are several reasons why the area adjacent to the line might be left free of solder resist, which was uppermost in the mind of the designer is a matter for speculation. In no particular order ...
a) For the look of the thing. You can easily see the 'keep out' area, that you need to keep free of stray wires, rats nest components, blobs of glue or labels. It also looks nice, for much the same reason that you'd leave a carbon-fibre component with its cool black weave on your motorbike visible.
b) RF performance. Solder resist in this area would change the RF performance very slightly. Whether it's enough to be significant depends on the operating frequency. Those connectors have ground tabs in a position that suggests it's nowhere near their RF limit, so I think the effect would be pretty minimal.
c) To allow easy mounting of components for attenuators etc. The beauty of CPW, whether above ground or not, is that shunt components like resistors for attenuators, diodes for switches, inductors for dc return to ground, or 6pin SOT363 components can be mounted between the line and grounds. The same goes for the ground of any coaxial probe. Much easier if you don't have to scrape solder resist off first before adding your own circuitry to the board.