Because he Begged, and swore on his life that he would reform if we gave him a second chance on our crew. You'll remember, if you promoted him, that he was tricked into doing some of those things that he did. (being intentionally vague, so that no spoilers happen)

Erin The Buccaneer on May 2, 2015 wrote:

Ahoy (:

I made a new person 'Belle De Bouff' and she's level 17. Well when I was level 12, I think I was on Ratbeard's 1st promotion quest and after I got the sword for Ratbeard, Lasko appeared and said something like this
"I told you to catch Ratbeard not him on your crew!". I was confused and thought the same thing. I was thinking you could get him in the crown shop. Please answer why you get Ratbeard as companion!

~Love Belle/Erin

Erin- 41

Belle- 17

During "Gunn's Gold", you get Ratbeard into your crew after defeating Chumba Wumba and rescuing him from being a sacrificial victim to the Waponi Water Moles' volcanic ritual for their mythic volcanic sentient that doesn't appear to our eyes, which I know it doesn't, thank Bartleby. During Ratbeard's first promo quest, "The Rat Race", after you get his sword, Lasko appears & says, "You vile, black-hearted traitor! Poisoned your own crew and thought nothing of it, did ya? And YOU, Pirate, we had a deal! Ye were supposed to catch Ratbeard to stop him, but ye took him on yer crew?! How dare you?!". It's because Lasko made a deal to only have us catch Ratbeard and stop him from stealing Gunn's gold, however, since he reformed and was recruited into your crew after the Waponi Water Moles tried to toss Gunn's gold into their volcano, Lasko was very enraged that his deal was broken, or so he thought. That's where you have to defeat Lasko and prove to him that Ratbeard didn't posion his crew out of his own will by finding the true villain behind it, which you'll find out soon once you keep leveling him up. Does that help you?