Are you watching the same film ducks?
The command centre is providing cover for the escaping ships.
The Imperials cannot bombard the planet because of the shield generator so they land the space camels to take out the shield generator and ground troops to take out the the ion cannon which the being operated from the command centre.
Luke and his friends have to slow down the space camels to allow the Rebels enough time to evacuate as many people as possible, the command centre has to remain manned to provide cover for the evacuating Rebels.
We have no idea how many TIEs the fleet has or how many can be deployed from one ship (considering the relatively small amount deployed by a much larger number of capital ships in ROTJ I don't imagine they could send wave after wave) and how many were in pursuit but were not close enough to be seen?
The one thing we know is that the fleet which had been previously deployed attempting to shoot down Rebels over Hoth had been ordered to track, slow down and capture the Falcon but not destroy it.
If they fired too much and in the wrong place Vader would not be able to get what he wants, he either imagines or senses (depending on if you want to kick in the Leia sister retcon yet or not) a Skywalker on the ship and every Imperial officer and pilot knows what will happen if they accidentally got too good a shot in.
The action makes sense in terms of the story.
Why a beweaponed bounty hunter would get another bounty hunter, armed with a big space rifle to guide a robot, carrying poison bugs, to open a window with space lasers and hang around to deploy them to poison a senator and then allow a Jedi to hang onto the droid long enough to be spotted and then shoot the droid (not the Jedi) etc, etc, etc, makes no sense at all.