There are many reasons, but the first is that it may not be your money at all. You may be the income beneficiary and your children or step-children might be the capital beneficiaries. This means you’re entitled to income from the trust, but not the capital. The best way to check is to review the trust terms in the will.
Alternatively, he may have left the money in trust to protect you; to ensure you don’t outlive the money, to make sure you don’t have to worry about managing the funds in your later years, or from yourself, if you have excessive spending habits, any addictions, or simply an overly generous heart.
Another possibility is to protect you from future relationships. Even if you marry, your spouse can’t access the trust funds, nor will they be awarded any if you subsequently divorce.