Because without asking Vidura, he (Dhritarashtra) never framed any resolution. Knowing (full well) the evils of gambling, he was still attracted towards it out of his fondness for his son.
अपृष्ट्वा विदुरं स्वस्य नासीत् कश्चिद् विनिश्चयः। द्यूते दोषांश्च जानन् स पुत्रस्नेहादकृष्यत॥