On a platform like that you're using bitcoin to place bets on whether something's price will rise or fall. The particular thing you're betting on doesn't really matter because bitcoin is the only asset that kollider accepts.

That said, the things bitcoin traders want to bet on are things they are familiar with that have easy price apis. Ether's price is a natural thing to bet on from that perspective. I don't blame kollider at all for giving traders buttons to bet on what ether will do. The thing I like best about kollider is that in theory you can bet on anything and you only need bitcoin to do it. It's all just "I'll bet 10k sats that this number will rise/fall" and then they can give people lots of options for what they might like to bet on.