This issue has always been a hot potato in transfer discussions. I think the official reason why tenants have a vote but leaseholders don't is because transfer from councils to housing associations changes the legal basis under which tenants occupy their homes (i.e. from secure to assured tenancies), whereas the terms of a leaseholder's lease do not change with the new landlord and so the legal basis under which they occupy their homes remains the same.
Having said this, there have been direct implications for leaseholders in transfers in that if transfer means that the new landlord will do improvements to buildings, then they will expect leaseholders to pay a proportionate share for improvements that affect them. And of course leaseholders have an interest in wider community issues and most people would normally expect leaseholders to be considered as part of the community. But then is it right that leaseholders are able vote on changing the legal basis of tenants occupying their home? Or that they can vote to prevent tenants from ever getting their homes improved? Hmmm - not sure that there is an easy answer to this question.