Why is the ratio of TSFC at altitude vs. sea level different for hi-bypass vs. lo-bypass turbofan engines?
The factor affecting TSFC is speed, not altitude. Altitude is indirectly responsible because the lower density requires a higher flight speed.

Low-bypass engines have a higher exit speed, so the change in speed affects their efficiency less. High-bypass engines double their TSFC between static and Mach 0.8 and only the lower temperature at altitude reduces the difference a bit. If you look at the formula for thermal efficiency in this answer, you see that efficiency drops more over speed for high-bypass engines. Only the much better propulsive efficiency of high-bypass engines makes them overall better for airliners.