Server Fault is a volunteer community and has no special insight into the inner workings of the clouds. If you wish to know why, and make sure they understand capacity issues, open a technical support case with GCP.
First alternative zones to try are the others in the region, asia-south1-a and c. Then maybe other close regions.
Or try different instance types. Less CPUs per instance. Earlier generation CPUs. Vary the amount of memory. Adjust the number of instances to meet your needs.
Moving an instance across zones is more like deleting it and creating a new one elsewhere. An API to help with such moves is available.