Why is "Martenot" in “ondes Martenot” capitalised but not “theremin”?

Vaguely:

1. Theremins are slightly better known than ondes Martenot. Not being capitalised is the norm for English words, and like pebbles in a river words lose unusual features the more they are used. A similar thing often happens with genericised trademarks (those who call all vacuum cleaners hoovers regardless of manufacturer, would normally not capitalise it).
2. The name ondes Martenot more clearly refers to its manufacturer. Even if one didn't see immediately that Martenot was a surname, one would wonder "waves what?" and then learn this was the case. Conversely, one might not realise that theremin came from the westernised form of the name Termen / Терме́н.
3. Of the various names first used for theremins, all were lower-case, and some included its inventors name (whether the Westernised Theremin or the more directly transliterated Termen); thereminophone, termenvox, thereminvox. From the various suggested names (along with ætherphone in a few spellings), the final name theremin was arrived at less directly from Theremin than was ondes Martenot from Martenot.

I say "vaguely" because there's no hard and fast rule about how these things will turn out, though in this case the last point made the lack of capitalisation on theremin particularly likely.