ISDN was very, very popular outside the United States. Hell, Tokyo's payphones used to have ISDN jacks for laptop users.
Tadpole later became a vendor that almost exclusively targeted the US military, but it didn't start out that way. I imagine those ISDN jacks saw a lot of use by European and Asian customers.
I went to work for a US bank in London as a sysadmin in 1994 and pretty much the first thing they did was have BT install 2x ISDN channels in my flat. It was incredible speed with what felt like almost no latency back then. What a treat! Moved to USA (also to work for that US bank) and it was like "ISD what???"
Tadpole later became a vendor that almost exclusively targeted the US military, but it didn't start out that way. I imagine those ISDN jacks saw a lot of use by European and Asian customers.