Here's a list of a few possible reasons

• UI differences
• Features
• Not Microsoft
• Open Core
• Different CI/CD syntax

It's still git under the hood and shouldn't make a difference in most cases.

GitHub is owned by Microsoft

Gitlab is OSS, and you can host yourself

(Ik most don't and use the public instance) Gitlab has features that GitHub don't have, or are better at somethings even if GitHub have them

1. Selfhostable gitlab-runner, faster build and deployment
2. Better permission management for organizations ( per repo, branch, registry etc)
3. Groups, project repos can be in groups

^ Some reason why my organization use gitlab over GitHub, I personally use Gitea.