[philipkglass]

Offshore you can install bigger turbines, because sea transportation of turbine components is not size-limited like rail/road transport. Taller turbines help to reach wind resources that are stronger and steadier. Also, even at the same height, offshore wind resources are often better than they are a few km inland. Finally, offshore wind can supply power to densely populated coastal areas when there isn't room to build turbines on land.

The disadvantages are higher construction and maintenance costs; waves and salt water are much more challenging to materials than the ordinary conditions onshore turbine towers experience.

[edejong]

Perfect answer. Another reason, which kind of joins with the density of the population is the significant noise pollution of larger wind farms.

[throwaway613834]

Great, thank you!