Because very few people will use it, most probably only you, but its complexity adds maintenance burden.
More features to test, more bugs to creep in, harder to change stuff.
How often would this need to be changed? Or more importantly I guess, how often has the old one been updated? I've been using this one for a couple years now on my own, so I can't imagine many, if any bugs remain in it. But feel free to test it.
Another useless feature, for most people.
It almost sounds like you think having the user cancel and reopen the dialog is acceptable UI design... While I suppose that it works, it seems fiddly, feels clunky, and more importantly is disrespectful of your end users time in my opinion.
I have a feeling that you need scripting support for re-ordering targets instead of this monstrous dialog.
There you can implement the logic you want with just a single click.
Have you considered this option?
I still don't know why you have some many targets. You want to build against every version of your libraries?
A monstrous dialog? Advanced mode is a handful of buttons with a few check boxes... If the user can navigate their way through the project options dialog to even get to it then advanced mode of this is a walk in the park by comparison. And even then they have to turn advanced mode on to even see it. Otherwise it's just as simple as the original dialog with a couple of time saving options.
Why would I write a script to make up for an open source program's UI shortcomings? Wouldn't it be more beneficial to me, the project, and it's users to just update the UI and submit a patch? I mean it would be like writing a script to overcome a bug instead of just fixing the bug and then submitting a patch...
And yes I need all those versions, they wouldn't be there if I didn't... I often don't even know who I'm going to be working for tomorrow let alone which version of whatever library they use. I only work at my steady job part time. So to make up for the lost income I often take up side projects here and there. And thus that's why I like to have a bunch of releases spanning the last 5 years or so on hand. Because you have no idea often I hear "We have not upgraded to version x in 6 years because version y broke everything"...