The mercury, generally in form of a solid amalgam (depends on the specific bulb), fails to vaporize well due to the cold environment. This leads to only a mild glow by a
little mercury vapor and the starting gases, and it takes a long time for it to warm up. Fluorescents of any kind generally don't do too well in cold temps below 10°C (50°F).
Here's a video explaining this much better than me:
https://youtu.be/ZcH0j7tOrDA?t=281