Complex sentence: Today these casts are studied by researchers tracing the history of the Maya through their hieroglyphic writing, because the casts are in better condition than the original monuments, which have suffered some weathering and erosion since the casts were made.
Split into simpler sentences: Today these casts are studied by researchers tracing the history of the Maya through their hieroglyphic writing. The casts are in better condition than the original monuments, which have suffered some weathering and erosion since the casts were made.