Bhutto was quite special, she was the first woman to head a democratic government in a Muslim majority country. She was seen in the West as a champion of democracy and liberal values in Pakistan.
In 1988 Bhutto campaigned on a platform of economic liberalism and the free market and opposition to both the socialist policies of her Father, military rule of Zia, and Islamic fundamentalism of the IJI. By holding this platform she made herself an ally of the USA, and the USA put pressure on President Khan to appoint her as PM after the PPP became the biggest party.
She continued to portray herself in the west as a groundbreaker for gender relations in Islamic countries, and as a model of a progressive and democratic leader in South Asia. She also allowed the USA to channel money to Mujahadeen and Taliban in Afghanistan, who were then engaged in a conflict with the USSR.
In short, she was a friend of the USA and they repaid this by allowing her, for example, to build her image of the progressive and democratic leader by addressing Congress.