Even if Ontario could legally increase its PST by 2% - which would have been and still is the best thing to do so that the Province, which is responsible for most of the things important to citizens - the Conservatives made it so difficult politically that the Ontario Liberals couldn't have done it. Federal Conservatives attacked the provincial Liberals continuously, saying (falsely) that Ontario is a high tax jurisdiction and repeating "Tax and Spend Liberal" so much Canadians could have been forgiven thinking that this was the United States.
Still, Quebec raised its PST - and they have a harmonization agreement with the Feds (don't know if it is different from Ontario's somehow, though). It would take political will. But we all know that a sales tax increase is politically toxic in English Canada. Look at the Alberta NDP, where they are getting rid of a decent broad based tax - a health levy - on anyone making 50K +and instead are increasing corporate taxes and income taxes on people making 150K+.
The provincial liberals figured that they would lose power by increasing the PST, and that was potentially correct. Then we wouldn't be getting all of the infrastructure spending but instead income tax cuts. It's all about holding the tenuous political line.