The fundamental difficulty
Straight from the wiki:
Fundamentally, objects (instances) reference one another and therefore form a graph in the mathematical sense (a network including loops and cycles). Relational schemas are, in contrast, tabular and based on the relational algebra, which defines linked heterogeneous tuples (groupings of data fields into a "row" with different types for each field).
Converting linked tabular rows to graph structures is hard, and even described as the Vietnam of Computer Science.