Two reasons I believe are these: 1) Sammy was the Iceman - he was a stone cold killer who meticulously planned his hits. And when you're going to whack the Boss of Bosses, you want someone like that with you. In other words, you don't want to miss or it will be you getting whacked.
2) Sammy was a tremendous earner. If the hit on Big Paul was successful, then Gotti could get his hands on some of the money Sammy brought in.

I recommend you read "Gangland" by Howard Blum. It's an excellent account of how the FBI nailed Gotti, how and why Sammy turned, and has a riveting account of the Castellano murder.