Why is it more difficult to horizontally scale a relational database than a NoSQL database like MongoDB?

Is the main reason for the difficulty in horizontally scaling / distributing an RDBMS because of an adherence to ACID transactions? Is it the fact that multiple tables are so interconnected, or something else? My impression is that it’s mostly the ACID requirements, given that different nodes in a cluster may have different values at any given time. But heck, I’m fuzzy on how ACID even works. Conversely, why are some NoSQL databases so much easier to distribute? I don’t know enough about distributed databases to understand why one db can be easily distributed but the other cannot. Can anyone shed some light?