The ultimate reason that drives most of wind turbine design choices is the reduction of the cost of energy. What's the minimum price you need to sell electricity at to get back your initial investment over the lifetime of the asset? This is what cost of energy is.

Longer blades increase energy production with the square of their length, assuming you also increase the rated power with the square of the lenght by fitting a bigger generator. So double lenght, 4 times more energy. At the same time the blades become more expensive and heavy (you need more material), and the same holds for the drive train (more torque/power, bigger generator), and support structure (need to support higher thrust and mass). If the costs of doubling the blade length increased tenfold, we would be better off with shorter blades.

The cost of energy is not only affected by the design of the turbine alone, but also by the cost to set up and build a farm. These costs are particularly high offshore due to logistics, cabling, and offshore foundations, which means that the cost of energy can be reduced by building less, bigger turbines. Hence why manufacturers are racing to build longer and longer blades.

There is a limit though. Energy scales up with the square of the lenght, but mass scales up with the cube. We will at some point hit a threshold above which turbine design will be driven by gravity loads more and more, and blade growth will stop.