ShareDrop assumes you are in the same network. It's a big limitation.
Regarding SkyTransfer, we created it for fun and to prove that Skynet and Sia technology work and people can create cool stuff with it. We strongly believe in technology behind SkyTransfer (not the value of the coin in the markets). File can be downloaded at any time, you just need the link. If a portal is down, you can simply switch to a different one. Obviously there is much more behind it, but it makes no sense to dwell on these discussions.