That might be why then.

I got a Junior Suite with Club access with my CSU in both October and December. If they don't have that available (for whatever reason), I don't think they are obliged to give you the King Suite.
Whether that makes good commercial sense in low, low season, well ...

Have you tried calling the hotel direct? The IHG service centre will be limited by what the system says. The hotel front desk might want to be more flexible.

In terms of alternatives, you will know best what works for you. Enjoy your break either way.