J. Michael Straczynski stated that TNT executives were getting involved in the production of Crusade and trying to create a show with wrestling, action, and gratuitous sex and violence. JMS refused, so TNT halted production and showed no more than the 13 episodes already filmed.
The TNT Memo
08 September 1998 - Ain't It Cool News broke the story of an alleged TNT memo that expressed a desire for more sex and violence in Crusade.
Confirmation from JMS
19 September 1998 - JMS confirmed that the TNT memo did exist, but had been resolved to his satisfaction.
22 September 1998 - JMS announced that new sets would be constructed during an extended hiatus.
The New First Episode
08 October 1998 - JMS announced that a new introductory episode would be filmed and that the costumes had been changed.
10 October 1998 - JMS assured the fans that the changes in episode order, costumes, and sets would improve the show.
Explaining the Changes
11 January 1999 - Ain't It Cool News reported on how the costume and set changes would be explained in the episode "Appearances and Other Deceits".
The Cancellation Rumor
29 January 1999 - JMS responded to a TNT cancellation rumor by announcing another hiatus and urging fans to contact the Sci-Fi Channel.
The Cancellation Announcement
26 February 1999 - JMS announced that the Sci-Fi Channel didn't have the money in its budget for Crusade and that TNT would air the 13 episodes already filmed.
JMS' Fairy Tale
06 March 1999 - JMS told a fairy tale at MegaCon about why Crusade production stopped.
DreamWatch #58 (May 1999) - JMS reveals all about the cancellation of Crusade and confirms the TNT memo and other rumors in this print magazine. He even gives an example of a "reprehensible" script note from TNT. At print time, JMS was at odds with Warner Bros. and the Screenwriter's Guild over his desire to replace his "created by" credit with his pseudonym: Eiben Scrood. The full article is no longer available online. An excerpt can be found in the Crusade for Crusade FAQ. Look under "Why did TNT cancel Crusade?"
"TNT cancels Crusade before it's launch date..."
11 June 1999 - Rob Owen's article for the Pittsburgh Post-Gazette looks at what went wrong between TNT and JMS.
The Rest of the Story
08 May 2004 - JMS spoke at Hawthorne High School in New Jersey where he revealed that he had learned the whole truth about Crusade's cancellation. Apparently, TNT had learned that viewers came to watch Babylon 5 on TNT and then left. Meanwhile, TNT's core viewers left when Babylon 5 came on and then returned afterwards. Since Babylon 5 wasn't helping to build TNT's core audience, they engineered a situation in which they could cancel Crusade without violating their contract with Warner Bros. The applicable section is about half-way through the linked transcript.