It is possible that primitive IE never allowed w to come in contact with an initial velar, and that whenever this combination occurred, it was simplified. This simplification is best explained by H. Pedersen's theory, IF 22. 354 f., of the ‘Abneigung der “reinen Velare” gegen die Verbindung mit einem w’. For, as he explains, the pure velars or ‘uvulars’ were the farthest back of the k - sounds, and consequently would not combine readily with the labial w. This would explain the apparent loss of w where no other phonetic reason is present.
A Wed, study studied this question.
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