I think they want to get spellcasting and spells done properly first. Most classes, barbarian included have some subclasses that have acces to spells from other classes. Or have feats that copy an effect as a feature. Barbarian can ritual cast some spells for example.
My guess is they wanted all of the spells done first, so if they make those classes they can just link to the excisting spell effect thet have (maybe with a unique icon) and not have to do the same job twice, basicly.
Also some things arent working properly yet like the above mentioned 'wizards can learn all spells' or that you can just swap spells out at will. Which is not how dnd spell lists work