Why is Steins;Gate in the Beta Attractor Field?
The Steins;Gate World Line is not in the Beta Attractor Field.

While the final result of the Beta Attractor Field (World Lines 1% to 1.99%) convergence is WW3 which is fought over Makise Kurisu's time travel thesis, Steins;Gate (1.048596%) is the only exception because the thesis is destroyed (and thus time travel doesn't exist in that World Line).

At the beginning of the final chapter of the VN and episode 23 of the anime, Suzuha herself had told Okabe that the Steins;Gate World Line is not bounded by any attractor field.