Italy, for centuries, was the center of opera performance. Most operas were written inItalian, set in Italy, etc. As such, it was very popular was Opera houses elsewhere in Europe to design their houses in an Italian style. Here is French for comparison. You'll notice the familiar reds and golds with Italy, with the palette for the French is distinctly blue. This caught on throughout Europe, and when films started to replace theatre as a main form of entertainment, this remained the trend.

It also has to do with the fact that red is the first color we lose in dark light, so when the lights are dimmed, the theatre becomes even darker than with green or blue seats.