Jim Jones did not want people to leave him. He has always been really weird about that. Jones felt that he did so much for the people under him and if they left him, that was a total betrayal. He has always been like that, even in his childhood. People from his childhood would say that Jones would have funeral services for dead animals (some that he killed) and forced the kids to be locked in his barn until his "service" was over. These people stated that Jim was "a strange kid".
Jones used a lot of patented cult techniques to keep people in line, and that if people left, they would meet some misfortune up to death because they "were not under the protection" of Jones. But even in California, Jones would send out search squads to find defectors. Again, Jones hated people leaving him.
One must also know that Jones had thousands of followers, stretched out in several People a Temple churches. There just wasn't one church, and not everyone who were rank and file members went to Guyana. Just not room for everyone. There also was not a wall or fence surrounding the property in Jonestown. People could slip out and escape. To stop this, Jomes took everyone's passport, their money, and made false threats that there were mercenaries out in the jungle ready to kill them if anyone were to leave. Pretty powerful stuff.
The suicide/mass murder happened because if he allowed the defectors to return. Then maybe more people in the camp would say "screw this sh.t" and wanted out also. It was a perfect storm of all the people Jones hated and in his little mind were out to destroy him, the Federal Government, defectors, and the media. Jones was also nuttier than squirrel sh.t by then, using all kinds of drugs and narcotics and was not in his right mind at all. The People's Temple itself was not a horrible idea, just a bunch of poor people and liberals who have come together to forward a better society. If Jones would have died in 1975, the group could have done without him fine and kept their ideology and vision. Jones destroyed this. It was all about him, and without him, there is no People's Temple.