Reason why collection of debt contracted by spouses cannot be brought against surviving spouse

Dear PAO,

Jun and Claire borrowed P3 million from me last year in order to finance the improvement of their house. Unfortunately, Jun met his untimely demise last month. I tried to collect the debt from Claire, but she gave me so many excuses. I am contemplating suing her to recover my money, but someone advised me that the proper legal remedy is to file my claim in the settlement of the estate of Jun. Why is it that I cannot collect personally from Claire after the death of her husband?