Sorry for the late response!

As a previous poster said, in fact Hong Kong Island and the Kowloon Peninsula were ceded in perpetuity, with only the New Territories being the part leased for 99 years, though 'only' is perhaps a bit inappropriate here, as the NT constitutes over 85% of Hong Kong's landmass and is currently home to half its population. The reason for the 99-year lease seems to be due to the Germans and French obtaining similar leases earlier in 1898, which I discuss here. As for why the whole thing was returned, see this answer by /u/spiritof454.