There are a couple of might make sense. One is that, it is easier to lubricate and free them up when they are not up inside the track.
The other is that you might think that running down to the home supply store and getting new ones is the easiest course of action. It could be, if you can find ones that match.
The article 'Troubleshooting Pocket Doors' has all of the topics related to this subject. If this is not your problem, try this link.
Pocket Door Hardware Concerns
One thing you need to realize is that there are no standards for pocket doors. Every manufacturer has different configurations for the tracks, hangers and rollers on pocket doors. Even the same manufacturer may offer multiple styles or change the configuration over time.
The Warning here, is that the rollers much match the track that is installed in the pocket door frame. They are usually not interchangeable.
About now you are saying, 'OK, I'll just replace the track'. Not a good idea most of the time. Replacing the track will usually involve tearing a wall apart and not be worth the trouble. Try to work with the rollers you have, or go online and try to find matching ones (not impossible, just tedious).
Removing a Pocket Door
You cannot remove the rollers without taking the door off. To take the door off you will need to release the door from the hangers.
Different pocket doors will release differently. Some have a slide stop that you push back. Others will have a spring loaded catch that needs to be pushed back to release the door. See the article 'How To Remove a Pocket Door' for more information and instructions.
Some doors will slide off the hanger sideways, others will drop down. An older door may stick a bit, so be patient.
There is a retaining clip that holds the bracket, that is attached to the door, to the hanger. Release the retention clip over and the door, along with the bracket, will slide off the hanger. Pick up the door and angle it, working it out of the opening. When the second hanger is in the door opening you can remove the door from it.
Removing Pocket Door Rollers
With the door removed it is easy to remove the rollers. Roll the first one to the end and allow the wheels to drop down through the opening at the end of the track. Tilt the roller assembly downward, moving it forward until the second set of wheels drops down through. Repeat the process for the second roller assembly.
With the pocket door rollers removed, you can take measurements to find a matching set. You can also take them with you to the supply store to find ones that match.