What is the secret and wisdom behind the great severity regarding the danger of debt, compared to theft, usurpation, and taking people's money unjustly?

What you mentioned, that the Sharia was strict concerning debt in a way it was not strict concerning usurpation and theft, is incorrect. Rather, many texts came with severe warnings regarding that, along with the deterrent and restraining punishments legislated in this world before the Hereafter.

The Quran and Sunnah warned against theft, usurpation, and consuming people's wealth unjustly, and the Lawgiver established strict penalties for them. As for debt, it is not an aggression, but negligence concerning it may lead to the loss of people's wealth. Hence, the texts came to preserve their rights. And whoever borrows with the intention of repayment, Allah will facilitate repayment for him. If he dies before that, Allah will repay it on his behalf and satisfy the rights of the claimants by His grace. Therefore, the Sharia is wise and just, encouraging solace and granting loans, and warning against negligence or malicious intent in taking people's wealth.