After knowing that most fluorescent tubes and non-integrated CFL lamps often use 4 pins and some non-integrated CFL lamps using 2 pins and an integral glowbottle starter, I have known that some linear fluorescent tubes such as the North American slimline fluorescent tubes often use only 2 pins so that they can be used on high OCV instant start ballasts. I wonder why the same 2 pin starterless instant start technology used in North American slimline fluorescent tubes was never applied to non-integrated CFL lamps.

It was.  During the mid 2000s several manufacturers introduced very fast-starting CFLi lamps.  They did not succeed partly because they were rather more expensive.  To instant strike an FL lamp requires either a high voltage (which costs more in a CFLi ballast), or an extremely high gas purity in the discharge tube to lower the energy required for ignition.  That also costs very much more, because lamps have to be exhausted more slowly than usual.  Moreover, the life is usually reduced.

The CFLi market used to be fiercely cost-competitive.  It seems that a majority of end users were not willing to pay the high prices for an instant-start lamp.  The reduced life claims vs standard types were also not well received.  The instant-start CFLi therefore disappeared quite quickly after its introduction.

I wonder why starterless instant start 2 pin non integrated CFL lamps that would use remote ballasts like PL-S and PL-C lamps were not developed as well.

It is my understanding that slimline fluorescents were much more than just two pins and an instant start ballast. The 8ft size in particular reduced the number of fixtures installed. Their higher wattage probably also meant higher efficiency. Maybe two 8ft tubes was also cheaper than four 4ft tubes?

Plus back in the early 50s, maybe rapid start systems were still not quite reliable enough, making the slimline tubes more appealing. But that's just a guess of mine LOL.