Why is beluga caviar so expensive when sushi is affordable?

I’m assuming you’re talking about Tobiko, a type of fish roe widely used in sushi:

Ikura, or salmon roe:

They’re not the same as beluga caviar, which originates from a critically endangered sturgeon.

Salmon is abundant. There are a lot of flying fish. Sturgeons, particularly the enormous ones that provide us with beluga caviar, are on the edge of extinction. (Other less endangered species of sturgeon caviar are available at a lower price.)

As a result, beluga caviar is exceedingly costly (and bad for the species). The rest may be had for a low price (and with as little guilt as possible for anything that requires killing animals).