Why did Pope Urban II issue the decree to fight the Muslims?
Neither of those, really. My usual answer here is to point to two previous posts:
A "Saturday Showcase" post on Why did the First Crusade happen? by u/J-Force
And several older answers in the Crusades FAQ.
/u/Ambarenya has previously answered Who started the Crusades?
/u/thejukeboxhero has previously answered Were the crusades an inevitable event, or were they purely the brainchild of Pope Urban?
EDIT: There are many redditors who study the Crusades from different angles and may have different answers than those linked above.
/u/WelfOnTheShelf has written a huge amount of answers on crusade questions, I'd recommend checking out the motivations section on their user profile https://www.reddit.com/r/AskHistorians/wiki/profiles/welfontheshelf