I'm wondering why we don't see many instruments made with Fir?
Because it wasn't what was used in the instruments that were popular at the beginning of the ukulele's mainstream success, and those woods which were popular early have dominated the market. I think that's really the only reason. The instrument wood market is pretty tightly focused on what has worked in the past. The "experimenters" using odd stuff are usually either small makers (like Beansprout) who have a low enough volume that they can get enough of a supply of whatever they want despite the market, or large makers who are using alternative woods for some specific purpose (marketing or economical) and have enough sway over suppliers that they can get what they want. Otherwise, everyone just sticks to what has always been used.
This produces a hierarchy where the most commonly used woods are the woods that were used most commonly in the past and it's hard for woods on lower tiers of popularity to become more popular. Unfortunately I don't think the hierarchy really accounts for actual tonal/structural properties at all. There are a lot of woods in the less popular category that are great performers. Fir among them. But to a certain extent, you can really build a good sounding ukulele from a huge variety of wood species, so there's often really no motivation for big brand makers to switch unless they are forced to.
So, basically, what it boils down to is that you don't see a lot of fir because it is hard to get a good supply of acceptable fir - because fir trees aren't really targeted for use in instruments. For a top, you really want quartersawn wood free from defects (pin knots, runout, twist etc) with tight ring spacing. For hardwoods this isn't so critical, but for a softwood like fir, it becomes pretty important - it's hard to make a top from flat sawn softwood and have it turn out nicely, because the stiffness and other characteristics just aren't there. This basically puts builders in a position where they have to be picky about their softwood tops. And this requisite picky-ness means that there's a sort of self-sustaining cycle where some softwoods are easy to get in instrument-quality lumber and some aren't - just based on what has been popular in the past, and not really based on what is actually suitable.
Sawyers will go looking for a red spruce tree big and straight enough to work for guitars, but they don't do that for fir. A lot of softwoods are grown and harvested for construction lumber, where they don't really care about growth ring spacing or small knots. Those tree farming methods produce a really high volume of lumber but don't really produce instrument-quality wood. A species like fir that's used almost exclusively as construction lumber is rarely if ever harvested in bulk outside these methods - so there just really isn't much quality, stiff, quartersawn fir around. To get the instrument quality wood, a sawyer has to go looking for it - which they only do for species that are already popular, like red spruce or sitka spruce or red cedar, etc. So for a species like douglas fir, there isn't much of a supply of the right stuff.
Kala or Martin could go out tomorrow and work with their suppliers to start getting a supply of fir, and then switch all their spruce-topped models to fir, but there's really no reason for them to do that. Manufacturers use what's available to them in the right volume at the right price, and what they're familiar with. This creates a perception in the marketplace that those woods are "the best" from a tone or structural perspective. The popular woods are certainly good - but that doesn't meant that unpopular woods aren't good as well.
