From the Jewish Encyclopaedia:
The avoidance of the original name of God both in speech and, to a certain extent, in the Bible was due, according to Geiger ("Urschrift," p. 262), to a reverence which shrank from the utterance of the Sublime Name; and it may well be that such a reluctance first arose in a foreign, and hence in an "unclean" land, very possibly, therefore, in Babylonia. According to Dalman (l.c. pp. 66 et seq.), the Rabbis forbade the utterance of the Tetragrammaton, to guard against desecration of the Sacred Name; but such an ordinance could not have been effectual unless it had met with popular approval. The reasons assigned by Lagarde ("Psalterium Hicronymi," p. 155) and Halévy ("Recherches Bibliques," i. 65 et seq.) are untenable, and are refuted by Jacob (l.c. pp. 172, 174), who believes that the Divine Name was not pronounced lest it should be desecrated by the heathen.