The libraries that were needed might well have been in the repo that you used to install aria2. But then you would have needed to install a lot more from that library – possibly causing problems for other software.
It’s best to treat your system as a package deal - a bunch of software components carefully selected by the opensuse team to all work well together.
If you want the latest version of everything (i.e. the “bleeding edge”), then install Tumbleweed so that your package deal is bleeding edge for just about everything installed. If you prefer stability, then stay with a stable release and accept that some of the components are not the latest versions available.