I work with KeePass2 on Linux (and Windows) voor quite some time. It's available for Ubuntu and LinuxMint, why isn't it in the AppCenter for ElementaryOS? We (=the familie & our small business company) really miss it!
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There are many applications not included in AppCenter. It will be endless discussing personal choices. The developers goal is picking software that integrates well in elementary environment. In any case if you find KeePass2 a must you can try the following link: https://installion.co.uk/ubuntu/xenial/universe/k/keepass2/install/index.html
Try using Synaptic package manager instead of App Center. It's a different program to manage the packages that the OS and the programs you use are composed of. In Synaptic you'll find Keepass2 and other stuff that is not included in AppCenter. To install synaptic, you can use AppCenter (just search for "synaptic") or Terminal by typing:
sudo apt install synaptic
Once you have opened Synaptic you'll find a search box at the top. If you search for keepass, you'll get a list of all packages that include that term, including KeePass2 and KeePassX. To install, right-click on the preferred package and choose install in the contextual menu. This will mark the package for installation. You can then search for other (unrelated) packages you might want to install too and also mark them. To finally install everything, you have to click on the "apply" icon at the top of the window. Then everything you have marked for installation will be installed.