While doing some research on lighting and collecting some lighting intended for the Japanese market, I have happened to find out that most non-integrated CFL lamps intended for the Japanese market use the exact same 4 pin G10Q base that circline lamps, 2D lamps, and FUL CFL lamps use while non-integrated CFL lamps intended for the European market, the North American market, and the rest of the world tend to use a different base type depending on the type of lamp being used. I would really like to know why the Japanese chose to use the G10Q base for most of their non-integrated CFL lamps instead of using a different base for each type of non-integrated CFL lamp.

A single common socket type eases the parts logistics, so drives down their cost. So if the users proved to be careful enough to not mismatch the fixtures "in the wild", the industry may get away without any keying against wrong lamp type.

In addition to that, I have noticed that each lamp also has a unique notch arrangement surrounding the G10Q base to allow the lamp to run in dedicated G10Q lampholders that help to minimize lamp mismatching. Because of this, in Japanese lamp catalogs, those G10Q bases for those non-integrated CFL lamps are officially designated differently depending on the lamp type such as “GY10Q2” for 23w FHP23 electronic ballast CFL lamps and “GX10Q2” for 13w FPL13 magnetic ballast with external starter/electronic ballast compatible CFL lamps.

Even in EU in the beginning some CFL prototype designs adopted the G10q base because it already existed from Circline.

However, in the 1980s and 90s the Europeans became somewhat obsessed with proliferation of cap-holder standards, because they discovered there was a lot of money to be earned from such diversification.  Possibly this happened because of the large concentration of lampholder manufacturers in Germany, who became very effective at generating rich profits by introducing new designs every few years and required luminaire makers to keep on changing to new and expensive designs.  The lampmakers fully supported this strategy for a few years, under the premise of also raising safety standards by eliminating the opportunity for misuse.  And luminaire makers also welcomed the frequent innovations because at each iteration, it helped to make the luminaires smaller and smaller and thereby more elegant for the enduser.

This era was a time when European luminaires became famous worldwide for being the most compact and with elegant styles, whereas America and Japan continued to produce relatively huge luminaires.  The same happened in HID - Europe swiftly adopted a plethora of pin-based lamp and holder standards to drive miniaturisation, whereas Japan and Americas remained far longer using large screwbase lamps.

If you look for instance at how long the bases are on Japanese Quad and Hex-type CFL lamps vs the European designs, its easy to appreciate how the EU designs permitted much more compact luminaires.  This is quite surprising, because in most other areas, the Japanese have the reputation for miniaturisation!  But not necessarily for aesthetic excellence, and that is what drove the Europeans to develop so many different CFL fits. 

These days it is recognised that Europe went too far though.  Especially with all the different keyways matched to each lamp current.  As Medved righlty says, the large array of non-standard types resulted in small production runs and expensive lampholders.   It was good for the profits of the manufacturers in the beginning, but not necessarily for the end users!  Or for the manufacturers later, when China swept in and undercut their costs far more easily than in other global regions.

They keying on G24q/G24d PL-C/T lamp holders is a good idea to prevent people who are unaware from putting the wrong lamp in a fitting, it would not be ideal if someone accidentally fitted a 10w G24q PL-C lamp into a 42w fitting!
The keying can be quite easily removed from most G24 lamp holders by being cut away which makes replacement lamp holders easier to acquire, as long as a competent person is servicing the fitting it is not a huge problem.

I also notice keying on the Japanese G10Q base CFL lamps to prevent people from accidentally installing the wrong lamp if certain forms of G10Q lampholders are used.