Why is it so common to use rosewood on the fretboard?
One factor is probably the influence from guitars. From a supply side, buying undersized (for guitar purposes) pieces from guitar suppliers is cost effective (relatively speaking). Something too small for a guitar is often big enough for a smaller instrument like the ukulele which means ukulele builders have a ready supply of instrument quality materials used by guitar manufacturers. Thus what is used in guitars can be readily used in smaller instruments. Win-win for everyone.
It is a hardwood but what about using koa especially on a koa uke?
It has been done for decades and it is still currently being used. I have a K brand ukulele that belonged to my grandmother which had a koa fretboard which showed a lot of wear and tear from repeated fretting of the strings against the fretboard. Is it still "playable"...yes. Can this be somewhat avoided...yes. It depends if this is something that "bothers" you or not but we as buidlers (and consumers) have a choice. Personally I would like a wood that is more durable (shows less wear) in this specific regard understanding that there are many other factors/considerations such as those that Allen mentioned. There are tradeoffs. That image of my grandmothers ukulele has made this one of those tradeoffs that I personally don't want to make. Not suggesting that one way is better or that there is a right answer just giving insight into the factors that affected my thinking on the topic. Others may value the aesthetics or some other factor over "durability" and that is fine.
just wondering what dictates the material used on the fretboard.
Cost, marketing, branding, durability, color, aesthetics, stiffness, resonance, economy of scale, if the builder mills their own wood, if the builder outsources milling, if the builder buys already milled pieces from another supplier. One cost and economy of scale consideration is that manufacturers that mill large amounts of Koa will, through the milling process, acquire "offcuts" (sizes that are too small for one part but may be used for a smaller part). If the milling process yields an "offcut" piece of 12" L x 2" W, that can be used for something like fretboards. In this context, the wood can be considered to be "very cost effective" as it cannot be used for backs or sides and would otherwise be sold as scrap or used for other purposes. So a manufacturer has the choice to 1) use the koa offcut for fretboards or 2) purchase some other wood and sell or use the koa offcuts for something else.
It might be useful to ask two separate questions, "why do large manufacturers of ukuleles use koa fretboards" and "why do small scale producers use koa fretboards?" The reasons/considerations are probably very different for each. Many layers to this one and I don't mean to suggest it is as simple as economy of scale. Marketing considerations probably have a lot to do with it too (ie: All Koa ukulele, made in Hawaii, with all Hawaiian wood) sounds very different than a (Mostly Koa ukulele, made in Hawaii, with mostly Hawaiian wood). Use a koa fretboard and neck and "mostly" easily changes to "all" in those sentences. Put economy of scale and marketing considerations together in this context and its a no brainer.
