That was seen as providing a better recognition of income, a truer picture of income, with the idea that under the tax system—and you can look at the IKEA and Canderel Supreme Court of Canada decisions—the goal is really to provide the truest picture of income in any year to determine the most appropriate base for taxation. The ability to elect to defer income until a bill is sent out under billed-basis accounting was seen as a deviation from that ideal system.
As I understand it, it was done in the 1980s in part because of the different regulatory landscape of the time and in recognition of the fact that back then the enumerated professionals weren't able to access a lot of the benefits available through incorporating their businesses. Of course, that's not the case any more, so the original justification for excluding those professionals from the general rule, which provides the truest picture of income, is no longer available, and it was considered to be inappropriate to maintain the election.