No idea, but a guess would be the need for pennies far outweighed that for halfpennies [and farthings], the requirement for which could be considered already served by the numbers circulating, whether genuine or not. Another angle could be that the security edge wasn't sufficiently developed for a currency issue, bearing in mind there was no security edge on the pennies or twopences. Or maybe it was down to a shortage of copper given the quantities needed for ships' hulls.
The IOM being self determining could presumably issue what it liked. The head of state might have been Victoria, but the Tynwald passed the laws.