because if they're hit with 50% tariffs their products won't sell and Lesotho won't earn any USD, which they need to pay for other foreign goods. The impact of that is immediate.
If they had 5 years until the tariffs were implemented they'd be able to adjust to the loss of income. Same as any sudden job loss causes issues immediately to your finances.
The insane part is since the tariff is calculated incorrectly, is one of the highest, and since they're so poor it affects them far worse than most other nations. They're also so poor and small that Trump mocks them, and wouldn't give them the same pause/change since they're insignificant at the negotiating table. So a possible collateral damage country here in the war.