This is because issuing a one month bill has little attraction to a bank: its liquidity cost is relatively high as its maturity is likely to occur within the 30-day liquidity window. Hence a bank would need to hold HQLA of similar size to the amount of funding the bank bill raised.
Instead, there is a greater incentive to issue at longer maturities and so the spread on 6-month bills has widened by more as there has been greater supply of such paper.