Chests need to be available through IP as well as RP.
When Chests were first released, a lot of people had thought that they'd be like runepages. Being 6300IP for each Crate. Since Riot was stressing that these crates are all for free. However. As the crates were released, people were a little disappointed that the free crates were only free through a limited 4 crate limit, that refreshes every week.
While this is a really cool concept, this still doesn't feel as "free" as we had hoped. However, buy making the crates cost BOTH RP and IP, it makes it a little more fun to grind out games. Especially for those who have nothing to buy with IP, those with 150k IP.
Yes. It's very possible that those who have that much IP will be able to buy a ton of crates. But it'll give them something fun to do with the IP they've been hoarding for the past two seasons.