Back in the 60's, the Justice League of America was the hot title for DC characters to appear in. During this time, they had a number of guest stars that joined the league. Yet in all that time, the Doom Patrol never guest starred. Check out this timeline:
1) Editors weren't used to the idea of books crossing over.
2) Having so many characters in a single title probably sounded daunting for both the artists and writers
3) The JLA editors probably just thought of featuring guest stars that were perspective members
For whatever reason, the Doom Patrol wouldn't end up guest-staring in the Justice League of America until 2004.
This was a lead in that introduced the team to a new generation of readers. In a perfect world, it would have given John Byrne a perfect kickoff for his take on the team.
Since then, there have been a few other attempts to revive the team, but they haven't been very successful either. Now we have a new version coming out written by Umbrella Academy's Gerard Way:
Doom Patrol is a group that you can use to tell stories that might seem out of place in your typical superhero comic. I think that's a core trait to the team that has survived even into this jaded age.