Why vlan.dat is saved in Flash Memory not in NVRAM in VTP Server Mode?

In VTP Server Mode, switch saves VLAN configuration information in a file named vlan.dat in Flash Memory.

In VTP Transparent Mode, switch saves VLAN configuration information in a file named vlan.dat in NVRAM.

In VTP Client Mode, switch saves VLAN information in RAM only.

I understand that switch can only save VLAN information in RAM only in VTP Client Mode. My question is why in VTP Server Mode, switch saves VLAN configuration information in Flash Memory not in NVRAM. My understanding is that compressed files such as IOSs are saved in Flash memory and uncompressed files such as startup-config files are saved in NVRAM; Startup-Config files are saved in NVRAM because NVRAM is faster than Flash Memory.

Can anyone explain to me why?