Hi Base,
this is something we discussed some years ago at Wiki Loves Monuments. Feel free to make another decision, but this is the rationale: It is common knowledge from fundraising that for every choice that you offer, you loose people.
Wiki Loves campaigns are aimed to attract new users, people who have no or little experience with Wikimedia. The copyright license is probably one of the hardest things to explain - and I'm already quite satisfied if they understand the basic idea of a free license, and if they are willing to release their pictures under it.
That is why we chose to only offer one license. We expected that the only people who may wish to pick another, are experienced Wikimedians - and they don't need the upload form per se, they can just use their usual upload method and add the templates manually.
Hope that explains! Best,
Lodewijk