It was sync engine reasons. That's why they're coming up with what they say is a better replacement.
That was one of the reasons in addition to the placeholder size and the reasoning that the placeholders were confusing to consumers (how stupid can consumers be?). They should have simply waited until the replacement solution was ready, nixing the placeholders is taking away a significant portion of function.
I read recently that October or so is when they will add OneDrive improvements, although no specific mention of what will be improved. I'm hoping that they put the placeholders back in. From what I read they are going to merge OneDrive for business/SharePoint with OneDrive personal. I've never had many syncing issues with OneDrive personal, but I use OneDrive for business/SharePoint and I must say it's a steaming pile of you know what. Endless sync issues, poor tech support, it's a nightmare. I hope they aren't moving over OneDrive personal a SharePoint system.