For more information about why the change to Conda was made, please read The Fermitools and Conda.
The FSSC has also created a Docker container pre-loaded with many of the necessary tools required to do Fermi Analysis. It includes the Science Tools, the HEASARC FTOOLS, Python 2.7 and associated libraries along with a host of other programs. This container will run on Windows, MacOS, and Linux. You can find it (with instructions) on github or Docker Hub.
The previous version of the Science Tools, v11r5p3, released Feb 15, 2018 is still available for download.