Treatments based on human biology are going to affect...

Treatments based on human biology are going to affect individuals differently. Some will respond to one treatment better than another, and differently at different disease stages. Tumors fall into sub-types and again, one group may respond better to one treatment over another.
Then there are the differences in regulatory regimes. It's a simplistic example but when I run out of migraine medication in Australia I'm given a prescription for naratriptan. When I run out in the UK I go to Boots and buy sumatriptan over the counter. Things happened in different ways in different places and the treatment of choice in each place was influenced by countless issues and events along the way. Then there's marketing.