Disposable diapers are bad for the environment because of the trees it takes to make them and the transportation and other manufacturing processes used to make the disposable diapers.
Disposable diapers are also made from oil for the plastic parts of the disposable diapers and trees for the fluff.
Disposable diapers also take a long time to break down. It's expected to take at least 500 years before a disposable diaper is decomposed in the landfill.
But cloth is not all that good for the environment either seeing as you must use soap, water, electricity etc to wash the cloth diapers.