>>11325029SPD is different from most Ranger teams because it's a law enforcement organization with proper training and rank and such. For a police organization to just grab two people off the street with zero training, and make one of them the leader of your new team over the people who have spent years training with that organization is kind of nuts.
The MMPR team had no formal training were randomly selected by a space wizard, who made the strongest of the group the leader. Enter Tommy, and Jason was no longer the strongest of the group. Even still, Zordon didn't appoint Tommy leader immediately after turning good. Tommy put in time with the group and proved himself. Even when operating on little to no power, he saved the team's ass again and again. Jason was also emotionally cracking ("Missing Green"), and Zordon may have known Jason was going to be selected for the Peace Conference soon after (he can see seemingly anything on earth with his viewing globe, so I doubt he didn't know it was coming in advance). Appointing Tommy leader when he did could have been done to build the cohesion of a Tommy lead team before Jason left, while still having Jason around for a while just in case Tommy didn't rise to the occasion.
The Space team is very simple. Space was Andros' territory, and they were using his ship, powers, weapons, etc. It would have been silly for TJ to have come in and lead that team knowing nothing of Space, just like it would have been silly if Andros had come to earth a year prior and become the leader of the Turbo Rangers.
Jen consistently lead the Time Force team, and while Wes did show leadership qualities at times, a lot of that stems from the Red Ranger having been designed to be the team leader. So for instance his Zord is the torso, and he gets the center Megazord position by default. The Time Force officers were also on Wes' turf in 2001, so they were fish out of water, and that allowed West to take charge of some situations.