A few reasons immediately come to mind.
The high bypass turbofans are significantly quieter, both internally and externally, than the early airliner turbojets or even low bypass turbofans like the RR Conway in the VC10. So no longer a benefit from mounting aft.
With that big mouth less likely to be subject to foreign object damage from material flung up by the undercarriage.
Weight. While more powerful modern engines are heavier. Think of that extra weight right at the back that needs to be compensated for.
Easy maintenance. Much day to day servicing can be done on the ramp from ground level on modern underwing engines. No need for platforms to get to the tail mounted engines.
The last is probably the most significant for cost conscious airlines, where the budget airline is king, and where turnaround times are kept to a bare minimum. Aircraft utilisation is significantly higher than in the 1960s and ticket prices lower. An aircraft stuck on the ground earns nothing and costs the airline hard cash in parking fees etc.