The latest version of Raspbian Stretch removed Mathematica from the Raspbian downloads list, as is evident in their release notes:
* Removed Mathematica
As confirmed by The Raspberry Pi Foundation, it was removed because the Mathematica contract expired after 5 years and was not renewed.
If you still want to use Mathematica on the Pi, you're in luck! Mathematica maker Wolfram Tweeted that Mathematica+WolfLang will continue to be available on Raspberry Pi:
The Wolfram rep went on to say:
Wolfram Research is committed to support Raspberry Pi's mission to put the power of computing and digital making into the hands of people all over the world.
This short guide will show you how to download and install Mathematica for free onto your Raspberry Pi, getting around its removal from the Raspbian downloads list.