T10 fluorescent lamps exist (1.25" diameter). Perhaps the most common use that I have seen is in 40W 48T10 germicidal tubes with pretty much identical specs to 40W 48T12. Not sure why they feel the need to reduce the diameter, but whatever. I think I saw here that some 48T10 Japanese energy saving lamps were also made.

Why was T10 not used more? It is a very interesting size, I don't think I would be able to identify it in the wild. Anybody have some T10 examples they would like to share?

T10 was a fairly recognized alternative to T12. There were a number of triphosphor F40T10 fluorescent lamps available years ago were offered as a high brightness replacement for the F40T12. There may even be some models still under manufacture today. These T10 lamps are pretty rare in the wild, but they are not hard to purchase.

There was also an F20T10. This lamp is pretty rare compared to the F40T10. Lights of America used to include them with some of their fixtures.

I have a couple of different types of F40T10. They are pretty common in Japan, to the point where F40T12 lamps were largely replaced by F40T10s there.

Anyway the couple types I have are:
Hitachi F40T10/EX-50/RS
Duro-Test Aurora V
Not technically a T10 but I have this T9 Power Twist lamp that's compatible with both F40T12 and F32T8 ballasts:
Duro-Test Classic 42 Power-Twist

I suspect the T10 lamps didn't really catch on due to additional cost and because they were harder to find compared to F40T12 lamps. F40T12 lamps were pretty available at every hardware store during the peak of fluorescent lighting, F40T10s remained fairly niche and had to be bought from a lighting or electrical supply house. Cost is a major factor too, it's probably why most commercial buildings that used F40T12s pretty much used the cheapest 34w cool white halophosphate lamps until they were no longer available due to efficiency regulations. 

Almost all aquarium tubes are T10, with the exception of 30w. On the packaging of my Hitachi F20T10 Aquarium lamp, it mentions F20T10, F40T10, but strangely F30T9.
You can also find T10s in Chinese factories, where they sell T10 and T11 tubes labelled as T12s.

Why was T10 not used more? It is a very interesting size, I don't think I would be able to identify it in the wild. Anybody have some T10 examples they would like to share?

Seems that T10 was some transient between T12 and T8. Without optimized fill (pure Ar like for T12) there will be a disadvantage of poor efficiency due to increased current density, with optimized (krypton mix) - a temptation to go further to T8 for even increased savings on phosphor and glass.

@joseph_125
Very interesting, I have heard of powertwist but somehow never noticed they were T9. Very interesting compatibility.

@LightsAreBright27
I did not know that, I wonder why they chose T10 over other diameters. Another example of T9 which I am just hearing about now lol. Never heard of T11 either, very interesting.

@RRK
Ahh I see, so it is just easier to go all the way down to T8 than to remain somewhere in the middle.