Hi,
I think s shaped arms became popular because they took a standard headshell fitting. This allowed ease of cartridge change. A s shaped can be good for resonances due to helping breaking up standing waves.
Straight arms have less mass for the same effective length & material. Also it's easier to get greater rigidity for a straight tone arms. Most of the one piece arms ( Rega & SME VI/ V) are straight. They are usually easier to make when straight & I think this is why they have taken off.
In the late 70s, there was a fashion for low mass arms with high compliance cartridges. This saw the rise of straight arms. Also arms like the Linn Ittok which had good rigidity came onto the market.
There are many things that are needed to make a good arm (rigidity, weight, bearings, construction, materials etc) and these (with careful design) can be done with either S-shaped or Straight tonearms.
SCIDB