I heard during a discourse that Sita performed sandhyavandanam and learned the Vedas. If women were allowed to do this in the other yuga-s, why did it not continue in the Kaliyuga? Pray clarify what put stop to this practice?

Compared to other yuga-s in Kaliyuga wisdom, capability, sincerity, purity, etc., are lesser. Hence the scriptures have excluded certain rituals.
For example, scriptures indicate tapas and yAgam are likely to be performed lesser in Kaliyuga. Similarly, they have indicated that ladies can avoid sandhyavandanam, etc., and chant stotras.