I would have to say that it's for the same reason that the tub, toilet, shower, and vanity sink ended up in the same room together.I've always wondered why do they make low voltage systems.
When low voltage systems were first being engineered, I'm sure that the people developing them never knew that they would be under an umbrella with similar technologies. They just knew that they were inventing something to fulfill a need. Then eventually many of these technologies ended up in homes.. and then someone figured that it made sense to group them together.
Like with the phone- 50 years ago nobody thought of it as being a "low voltage" device. Even the manufacturers of PBX systems were probably working towards an efficient design more so than the thought of specifically using "low voltage" to build their equipment. Then came cable TV and internet... then high speed internet.. so home builders figured that houses should have a "commercial" type of voice and data infrastructure.
Then from that point it began to snowball.