What's the reason behind cosmetics not being available offline?

If you're playing offline, then your game is not connected to your account. This means the client has no way to know what cosmetics your account might have, as it doesn't even know what your account is. There is two offline modes (offline and true offline), however they both need to work with the same code, which means the offline client can't support game features true offline doesn't. Things like chat and friends lists can work in offline (not true offline) still because they're more like a "layer" on top of the game.

We haven't given up looking at options to potentially make cosmetics available offline. However currently, the whole crux of it, that which prevents access to your account purchases, is the very feature of the game mode itself. Being offline, not being connected to remote services, such as accounts.

No, because then it wouldn't be true offline. That's a connection occurring. We had previously raised the possibility of having "check-in connections" every few months for offline mode, however the community feedback very strongly stated that is not true offline. So we agreed to the community's feedback and made true offline, truly offline.