Why was the Canonflex a "fumble?" I ask legitimately, as I am not familiar with it. Was it a poor design, a bad execution of a good design, lack of marketing, or something else?
Something else.
In the late 50s Canon was doing
really well with its rangefinder sales. Nikon had the vision to gable on SLRs as the future, while Canon had all the reasons to keep their focus into their rangefinder lenses and cameras.
The Canonflex was, IMO, a half-hearted effort by Canon. You take a look at some of the Canon R lenses and you even see lack of some fit and finish that IS present on the rangefinder lenses, like for example matte-black diaphragm leaves.
The Canonflex was launched with no wideangle lenses (!), while Canon used to have the fastest 35mm lens for rangefinders in the 50s. This gives you an idea of how much important was the SLR for Canon.
When you take a look at the Canonflex leaflet, you see there were many lenses, like the 85/1.8, that existed in rangefinder form (using a reflex attachment) but not in native R mount. You would mount it on the Canonflex by using an adapter.
Thus, Canon perhaps thought the professional would use the rangefinders for wide angle shooting and the SLR as an accesory for the tele lenses.
Nikon, instead, launched the Nikon F and in 1960 it already had the 28/3.5 H, which for its day was an excellent lens.
Only later, circa 1964-5, Canon started to take SLRs seriously, releasing significant lenses like the 58/1.2 (first f1.2 for slr cameras), the 19/3.5R (fastest and widest retrofocus SLR lens at the time), and truly pro lenses like the 85-300/5. 1968 it released the legenday fluorite telephotos, 1969 the prototype FL 55/1.2 with aspheric lenses was released , and 1971 the FD line is released together with the F-1. You could say that from 1965 or 1966 they realized they had to put their money on the SLR system.
EDIT: I had the chance to buy a Canonflex long ago. It has very nice build quality and heft. The viewfinder is just ok, not better than the one in the Nikon F. The mechanical operation is very smooth. The bottom-located wind lever is comfortable to use. The viewfinder is far easier to remove than in the Nikon F.
However, it was going to be doomed to failure, because, in 1959, compared to the Nikon F, the canonflex had:
- No motor drive
- No interchangeable screens
- More limited lens selection, and even more limited amount of automatic diaphragm lenses
Furthermore their choice of automatic diaphragm was mistaken; they chose to have lenses that require to be cocked so the diaphragm is released by the camera body. This opens up the possibility of inserting a lens that isn't on the 'cocked' state and thus missing a shot by wrong exposure.
Canon had to revise the mount and create the FL line of lenses, in 1964, with a totally new diaphragm system.