Simply put, because of a few bugs the flamethrower worked fine, but wasn't nearly as epic looking or as satisfying to melt enemies with. Here's a side-by-side comparison of the two versions.
Old:

New:

I think it's pretty obvious why we changed this.
Full Changelog
- Fixed movement speed/direction of the flamethrower particles
- Fixed size increment speed of the flamethrower particles
- Fixed a bug where the flamethrower had no audio