Low Kappa score but high accuracy
I'm not sure that anything is wrong here. With a binary target, chance is 50%, and 75% is half way to perfect accuracy of 100%.  Kappa, which ranges from 0 at chance to 1 at perfect separation, is 0.4, which is again a little less than half way to perfect prediction.  So it seems to be corroborating the performance of your model.

Kappa is defined as:
(observed accuracy - expected accuracy)/(1 - expected accuracy)

Since your classes appear balanced, you have (0.75 - 0.5) / (1 - 0.5) = 0.5