What makes it so more difficult? The nature of the problem.

Ozone depletion was caused by using certain type of substances. The technology was developed so as to eliminate them and use others instead. Not big deal.

But climate change is at the core of how countries work and live, and what choices they made in the past and will do in the future in terms how the energy for the country is produced and used. Thus quite difficult to just get rid of everything that emits greenhouse gases in all countries at the same time.

It is not China's fault to have the largest coal reserves on Earth; or Canada to have tar-sands, or countries in the Middle East and the Pacific (Asia) to be sit on huge volumes of natural gas and oil. Those are their resources, what they have at hand, and although may seem obvious, what they would use as energy sources. Meanwhile, other countries, with not the same amount and type of resources, need to generate electricity and heat.

Who goes first? Who is brave enough to tear down all the electricity generation plants and substitute them with solar panels, or other technologies? Which country has enough resources as to do that without stopping the economy or interrupting people's lives? At which economic cost, paid by whom? And at the same time, eliminating any vehicle that uses fossil fuels. It is just unrealistic.

Climate change is, in some way, more a problem of disconnected timelines. That of the fossil fuel extraction, production and use; that of the emissions concentrating in the atmosphere; that of provoking an intensified greenhouse effect; that of increasing the temperatue and shifting the climate... and that we use for our decision making, our investments, our analysis, etc. It is not that the technology does not exist; it is not that the are out of economic resources to do something.

And the ideas might continue to further complicate the panorama. Does this answer help understand why?

To add to Chris's comments, another difference is the timescale and evidence base for acid rain/ozone vs climate change. The effects of acid rain (and to an extent ozone depletion) are pretty much instantaneous with immediate consequences, whereas there are still huge uncertainties with the timescale and impact predicted from climate change. The science are also much clearer with acid rain and ozone layer, with specific and direct cause-effect relation, unlike climate change having such a broad range of causes and indirect consequences (sea level, glaciers, weather patterns etc). All these, coupled with the costs, economic sectors and international diplomacy involved, makes it far more difficult to achieve an international consensus.