That would be the concertmaster. When music was often played without a conductor, it was the concertmaster who would lead the ensemble. They serve, musically, and sometimes organizationally, as the head or representative of the orchestra. In modern symphonic ensembles they come on stage separately to tune the ensemble. I'm guessing it's just tradition at this point.
As to why the focus and acclaim, I think it's a combination of a few things. The first is a very, very high skill ceiling. I've heard that French horn can be harder to play, but violin is definitely considered to be among the hardest. Then there's also repertoire. While there were early concerti for most instruments, it was even as early as Vivaldi where virtuosic violin rep started to appear. Then came Paganini and it was game over for the rest of the instruments. The sheer volume of pieces is why violin dominates over, say, cello. There may also be a physical size factor. You just can't cover the range of a cello or bass as fast as a violin, though I've seen some insanely impressive low string players.
As to why the focus and acclaim, I think it's a combination of a few things. The first is a very, very high skill ceiling. I've heard that French horn can be harder to play, but violin is definitely considered to be among the hardest. Then there's also repertoire. While there were early concerti for most instruments, it was even as early as Vivaldi where virtuosic violin rep started to appear. Then came Paganini and it was game over for the rest of the instruments. The sheer volume of pieces is why violin dominates over, say, cello. There may also be a physical size factor. You just can't cover the range of a cello or bass as fast as a violin, though I've seen some insanely impressive low string players.