Why this happened is a matter for speculation. Item of keris dress were often sold off or exchanged for various reasons. Economic distress etc. etc.
This we know.
What we don't know is why this blade was fitted to this particular sheath. There can be two possibilities.
One is that the dress was sold off to an "antique dealer" who refitted the blade for sale to a "tourist" or in this case, a museum curator it appears. Why this blade was chosen is a bit of a mystery. After all the dealer would stand to profit more by using a finer blade but perhaps he was relying or speculating on the ignorance of his clientele.
The other possibility is that this was a more "traditional" exchange where one Balinese sold the dress to another. Perhaps someone wanted to dress this particular blade in finer clothing but saw the second hand dress as a better value than commissioning a new set.
The only clues we have are the relatively careful and neat job done to re-fit the blade, which was not always done, particularly for tourist keris. And of course, the blade itself.
I would contend however that there is more than meets the eye with this blade. It is not a "bad" keris in the normal sense of the word IMO. It has clearly got some age to it and has been well cared for in the normal way. The unusual aspects of this keris include the ganja iras and the curious "pitting" effect of one type of metal on the blade paticularly around the dapur area.
You must also understand that traditionally, ganja iras was neither common nor undesirable. Real, old keris ganja iras are quite rare although certainly not impossible to find and were considered to have special magickal properites. It is not just a "cheap shortcut" way of making a blade.
You must realise of course that I am not "defending" this blade on aesthetic grounds, though to an extent it does have that certain something that nice old blades can have when they're well looked after. What I am against is the notion that just because, to our Western eye this keris is not up to the standard of the dress, that this was necessarily so for the Balinese.