TL;DR: People who oppose legalization in favor of decriminalization are unknowingly sentencing those oppressed in other countries by cannabis prohibition, to an extended period of oppression.
Cannabis prohibition is a global scourge. Countless people die every day in countries all across the globe, both developed and developing, as a result of organized crime and a rabid police state. A decriminalized market in America means the oppression stops here. The influence decriminalization would have on other countries would be diluted or reduced in efficacy, compared to legalization.
The most powerful advocate for the end of global cannabis prohibition is a massive customer base that has significant experience with the product. This experience is far easier to impart when the product is easily accessible price-wise and distribution-wise. This experience is far more impactful when the product is consistent and effective.
Does anyone think that a decrim-only market is going to produce cheap, consistent, and effective product? Remember that most people don’t want to bother with cultivation for personal use.