When Harry was growing up, the focus for Sirius and Remus was on helping him understand his parents as loving individuals who sacrificed themselves for him rather than delving into their past relationships and friendships, especially with someone like Snape. The primary goal was to foster a positive emotional connection between Harry and his parents, without clouding the image of Lily and James in Harry's mind.

Secrecy and Trust

The Marauders had a history of keeping secrets, particularly regarding their own pasts and the events surrounding Voldemort's rise. Revealing more about their past, including their relationship with Snape, could potentially complicate things further. Sirius and Remus might have felt that the truth could reveal too many uncomfortable truths and could lead to more conflict and distrust.

Lack of Context

By the time Harry learns about his parents' history, the full context of the relationships and the choices made by those involved may not have been clear. Sirius and Remus might have felt that they couldn’t adequately explain the nuances of Lily and Snape’s friendship without oversimplifying or misrepresenting it. The lack of definitive information and the potential for misinterpretation might have been a significant factor in their decision to keep the information hidden.

Conclusion

Overall, Sirius Black and Remus Lupin’s decision to not tell Harry about Lily and Snape's friendship seems rooted in a desire to protect Harry from unnecessary pain and confusion about the complex relationships of the past. Their motivations were driven by a combination of protecting Harry's emotional well-being, their personal loyalties, the complexity of their own memories, and the potential for adding more burden to a young and already emotionally fragile boy.