He's out of contract this summer and Rov are offering him three years on a hefty paycut. He's probably waiting to see if anyone who can pay more comes in.
Rotherham are hopeful of making Towell a permanent signing within the next 48 hours: https://www.thestar.co.uk/sport/football/rotherham-united/rotherham-united-millers-hoping-to-turn-loan-star-s-deal-permanent-before-end-of-month-1-9543627