Why do some deserts have sand dunes while others are mostly bare rock or gravel?

A desert is defined by low rainfall, not by sand. Sand dunes form where wind has abundant fine sediment to transport and deposit, often from ancient riverbeds, dry lake beds, or eroding sandstone. If the landscape is dominated by exposed bedrock or gravel, wind cannot easily build large dunes, so the desert looks rocky or barren instead. This is why places like the Sahara include both dune seas and broad stony plains. The local geology and sediment supply matter as much as the climate.