My bet is that it has nothing to do with any variations of Covid.
About now most people will have booked their Xmas and New Year holidays. Eva would have looked at the figures and probably decided it was uneconomical to operate these flights.
Looking at recent figures published for flights and passenger arrivals at Savanabhumi it seems load factors are very low indeed.
Thai procrastination, plus their horrendous bureaucracy, has probably put many off going to Thailand.
They are going to learn to regret it.