Hey there.
It really comes down to a tax differential. BEP.UN issues a distribution which may include things like interest, dividends, return of capital, foreign income, etc. Whereas BEPC issues strictly an eligible dividend. They made this change to be more attractive to US investors and to those holding in non-registered accounts as they are much easier from a tax perspective.
Of note, they are considered economically equivalent, but the share price fluctuates based on demand.
So basically, it comes down to how the distribution/dividend is taxed. BEPC is more tax-friendly.