Why is it better for astronomers to measure parallax by observing every six months instead of observing every year

The baseline is longer

Explanation:

Each year, the Earth is in the same place relative to the sun, give or take a quarter million miles. Hence the sky pretty much appears the same.

In 6 months, the Earth is about 2 AU (about 186 million miles) from where it was, so the baseline for observing parallax is much longer.

The longer the baseline, the better, when measuring parallax.