This is because paintings from Japan increased their reputation being exported to Western countries after the end of Edo period, started to be called 'Japanese paintings' to distinguish them from the traditional Western paintings, and likewise, the term Japanese sword in contrast to Western swords became established.
その理由として、幕末以降、西欧に流出して評価が高まった日本の絵画が従来の西洋の絵画と区別して「日本画」と呼称されるようになった事に対応して、西洋の刀剣に対して日本刀という呼び方が定着した。