That is one of the holdout artist's from adding their classic material to streaming services. Spotify users have been waiting on tender hooks if Def Leppard will ever add their classic material as well.
In the meantime this is the information that is suggested to be shared with other users:
Spotify wants all the world's music, and they're working hard on making this happen for you. However sometimes agreements can’t be reached with the artist or label or a change may happen in music ownership.
Spotify adds thousands of albums and artists are added every day. If you can’t find an artist you’re looking for, they may appear (or reappear) soon. In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify.