Generation Pennywhistles are robust and have accurate tuning with long tradition behind them made in the UK

Why do players prefer the key of 'C'?

By learning in the key of C one is emulating the notes of a flute to large degree so its the best one to start on if thats your possible later aim.

secondly it can help learning  to read as one starts reading music in the key of C major./ the most common key by far in music. (see chart below)

Also the penny whistle in C is the longest one so gives the additional whispy lovely notes in the lower range besides the piercing higher  range

range