Same way Google and Facebook are hit by the reduction in advertising spend. Twitch is, from a business perspective, an advertising company. The streaming is a means to that end, just as search or social media is for Google and Facebook.
And their other side is discretionary spending. Probably of the lowest order as many are likely to pick other discretionary things over donations to streamers. How many will pick donating to streamer instead buying maybe a cheeseburger or some other treat...