Allah سبحانه وتعالى says in the Holy Qur'an:

أَلَمْ يَأْنِ لِلَّذِينَ آمَنُوا أَن تَخْشَعَ قُلُوبُهُمْ لِذِكْرِ اللَّهِ

"Has not the time not come for the hearts of those who believe to be affected by Allah's Reminder."
[al Ḥadīd 57/16]

In order for a person to be to be fully receptive of the laws of Allah سبحانه وتعالى, his heart and mind must be prepared; tazkiyah enables one to do this, just as a farmer prepares the soil before planting the seed. Rasulullah صلى الله عليه وسلم reformed the hearts and minds of the people in Makkah over a period of 13 years without any ahkaam (laws), so when they reached Madinah they were ready and willing to accept the laws of Allah سبحانه وتعالى that were revealed there.

[This short excerpt is based on a brothers' majlis (gathering for spiritual training) held by Shaykh Abu Yusuf Riyadh ul Haq on 7th June 2000]