Why is ethereum not using threshold signatures like dfinity for randomness?
I'm assuming you're talking about Ethereum 2.0 with PoS & Sharding. Think it's due to the differences the design goals of these two networks - Dfinity favors consistency over availability as explained in their consensus paper (see page 2). Ethereum on the other hand, seem to favor availability and Justin Drake, in this post seems to suggest that RANDAO in conjunction with VDF suits their principles.
Personally, I think Dfinity's impetus on consistency makes more sense, especially since we are talking monetary systems & enterprise adoption.