You're right, that is more realistic, but this is alternate history, so do we need to constrain ourselves?
If we don't, then anything is possible and discussion shifts from
why things went this way (and how those factors could instead lead to different outcomes) to
let us make things go another way (regardless of plausibility).
Such discussion tends to be inherently poor because we either backtrack a lot to change stuff before the events and allow for the desired change, which can be a devil of its own (would a less racist USA, able to incorporate Liberia, actually create it in the first place? chances are it would not) or we're making things happen despite the changes (butterfly net) which quickly turns out very poor as different events poorly mash with leaving everything else running 'as expected' (the State of Liberia partaking in the Civil War that happens regardless, on the same timeframe, despite race relations necessarily evolving differently from the prerequisites, let alone the consequences, of the proposed change).
It sounds like a constraint only if you want to forcibly make X happen, otherwise it's more akin to a fun puzzle where you have ingredients, but attempt to reorder or swap them or, where possible, change them to come up with a different historical recipe.