Why are ubuntu mainstream in the 14th and 16th series?

The LTS series 14 and 16 are supported for 5 years, and the other 15 are supported for 9 months.This means that if you use the 15 series, you will have to upgrade within 9 months, but for server use, this is troublesome.
_{(That's why 15 series was used for desktop use.)}

Also, if it is for server use, I think it is installed as Ubuntu Server, but the Ubuntu Server download page recommends LTS version even if there are non-LTS versions newer than LTS version.

ExampleUbuntu Server Download Page When 15 Series Was Released

In any case, the non-LTS version will be talked about for six months to nine months, so I think LTS, which has been in use for five years, has more information online.

Maybe it's because 14 and 16 have LTS and 15 don't have LTS.
LTS releases every two years.Also, the normal version of the support period is 9 months from the release, while LTS (14.04, 16.04) has 5 years (although the normal version of the 14.10 series has expired...)

https://www.ubuntulinux.jp/ubuntu