why so many programmers are using this ?

because they don't know VIM, of course :)

1. Integrated terminal. No need to switch between windows.
2. No need to use the mouse.
3. Pretty standard editing workflow: you can use block selection, copy-paste and stuff, which is not the case for vim. UPD: I mean that all these things can be performed in a way similar to other editors (Word, Kate, Notepad, IntelliJ IDEA).
4. Integrated file manager. I hate navigating with cd && ls, I prefer to have everything on the screen.
5. Very good intergration with terminal: I can navigate with cd when I want, I can use panes, I can switch current directory between panes, I can immediately open and edit any file right here, I can run any commands without re-typing file name over and over.
6. Stability and speed — everything is instant (except editing of huge files — they're fully loaded in memory).