Polar bears do not spawn

Polar bear will not spawn unless they were pre-generated with the world, once dead no new one will spawn.
Create a Superflat world with the following preset

3;minecraft:bedrock,45*minecraft:dirt,minecraft:grass;12

Kill all other currently existing entities

/kill @e[type=!player]

Wait some time
You will get rabbits spawning but no polar bears

If you create a normal world and then use something like mcedit to remove all blocks then just put grass down in the ice plains only rabbits spawn. Putting grass can be done manually or with fill command or making structure block for it.
For all above i recommend changing gamerules for daylightcycle and weather such you only dealing with polar bears and rabbits you can also setup a counter system with the command blocks