Because it's making a general statement about what a typical elephant is like, not one particular elephant. The meaning is closer to "An elephant is [a] big [animal]."
Either that or also accept "The elephant is big" as an alternative answer. I believe it's correct, depending on context - certainly in informal speech. Having said that, this course has really highlighted how lax my grammar has become, so I could be wrong - I alledgedly speak Hindi as a 2nd language at home (to English) regularly.
As होता is for generalisation... shouldn't "(the) elephants are big" be accepted too?
Not a hindi speaker but I think it wants to say "elephants are usually big"