This is because carbon forms long and sometimes complex covalently bonded structures and also because carbon compounds vary greatly in the type of interactions they have with water. For example some carbon compounds, such as most lipids, have strictly hydrophobic interactions with water, others such as sugars are hydrophilic. Still others such as proteins and phospholipids are partly hydrophobic and partly hydrophilic.
Diamonds form deep in the earth under extreme pressure and heat.
A diamond is essentially one large covalently bonded molecule as shown here. Graphite is soft but consists of many small flat crystals consisting of covalently bonded carbon molecules. Its the sliding of these crystals past each other that makes graphite soft and slippery. Fullerine gets its name because it resembles the geodesic domes developed by Buckminster Fuller. Fullerine also occurs in large balls and tubular fullerine structures have been made in the laboratory.