Well, back in the day it was "modern"...
Singing in The Rain was the Hamilton of it's day.
The way Hollywood stars now have to do stunts, the stars of yesterday had to tap. If you couldn't tap, you couldn't be in the MGM musicals. Or the variety shows in the local theatres or performance circuits. The big stars all learned to do it.
Over time it's been replaced with other styles.
It's never "gone away", it's just not seem as cool or popular enough to be mainstream.
It still goes on at dance classes all over the world. There are still tiny tappers of 3 and adult tappers in their 80s. Musicals like 42nd Street and Anything Goes still do well and have showstopping tap centric routines.
It's still a skill that takes years to perfect.
It's still a "street dance" in parts of the US.
But in terms of the dance world and mainstream performing arts, it's just not as favoured as other styles anymore. There are trends...at the moment its all commercial heels/voguing/pose classes or acro dance or socially bachata...contemporary/lyrical is more popular than tap.
Trends change. Dance evolves.
I'm sure, like Burlesque, it will make a comeback eventually. But maybe not for a while yet.
It will take a big Greatest Showman type of film or a genuinely amazing troupe on a talent show to make a big impact.