A couple of reasons. One, Keldeo doesn't need the extra power of HP ICe as the utility of lowering an opponent's Speed outweighs the extra damage, ESPECIALLY in the case of Lati@s, as they would be able to tank the HP Ice, outspeed and OHKO. Both Latias and Latios die to Icy Wind if they come into it when Stealth Rock is on the field, which HP Ice does not give you.
And second, because Keldeo really needs the other Hidden Power slot to hit other targets.
(16:08) <@skylight> BUT I DID NOTHING WRONG
(16:08) <@skylight> CHERUB
(16:08) <@skylight> i will FUCK YOU
(17:20) <@skylight> ill luvdisc your ass