@Angrybrony
In-universe: According to the Legends continuity, the ship originally belonged to a bunch of slave traders of a prison moon, where there were 6 of them each with the callsign of “Slave X”. Jango destroyed the other 5 ships before stealing Slave 1, and i think he just couldn’t be bothered with changing the name of the ship.
Out of Universe: Apparently it’s some sort of editing in-joke when it comes to makingcopies of a tape. The original tape is called “Master”, while the copies of the original were called “Slave (number)”, so that is apparently the reason for that..
Also like what Zincy said, Disney is not changing the name of Boba’s ship, this is just LEGO changing the name of it to help make it more marketable.
THe average parent who only has a casual knowledge of the franchise probably doesn’t know what their kid is talking about when their kid says “Can i get the LEGO Slave 1”, but they would know what to look for if they hear “Can i get the LEGOset of Boba Fett’s spaceship?”