I'd presume the most common are probably other experience with GCP, other applications on GCP, or prefer GCP tooling to Azure. I haven't compared pricing, so no comment there. Lastly, anti-ms pov.
For me there's main reasons are 1. Free tier 2. Features in that order
It is perfect for "hobby" projects that have no revenue today. If a "revenue strategy" emerges, it isofcourseeasy to enable billing but there is no 12 month $300 trial.