Because “Esq.” says “Hey! I’m a lawyer!” 
Actually, my business cards have “Esq.” on them, even though I never use it in correspondence or anything. I included it because my business card does not otherwise indicate that I am an attorney, as I work for the government and have a rather administrative-sounding work title.
Sure, but that applies here in Canada too. I know lots of lawyers in gov’t employ with job titles that don’t make it clear they’re lawyers.
As long as we’re on the topic, when did Esquire come into use for for lawyers? I don’t think Perry Mason was ever addressed as Esquire in the TV show, nor was anyone in most of the classic movies from the 20th century that involved lawyers, barristers, or solicitors. I first noticed it in the early 1980’s.
Esquire originally meant a member of the lowest rank of British gentry, or alternatively, someone who by education and professional achievement could be considered a gentleman. Samuel Pepys was some sort of high-level government clerk, and he was able to append ‘Esquire’ to his name. But as I noted above, its use now seems solely restricted to the legal profession, so much so that I was talking to somebody once who said they wanted to get their “E.S.Q. degree”!
hmm… I don’t know any practicing attorneys who don’t have “Esq.” or “Attorney at Law” or some other such indicator of them being an attorney on their business cards.
If I remember correctly Tom Sawyer referred to himself as Esquire.
My first question on thinking about this was, why didn’t they just use the degree after their names, JD, and perhaps LLM? But then it wasn’t so long ago that lawyers were often not degreed, but simply clerked or apprenticed with some established practitioner and then sat for the bar exam when they felt they were ready.
Don’t forget though: one can have a JD and a LLM and not be a lawyer admitted to practice 
And in Canada, every M.P. is “Honourable” and all Prime Ministers, current or former are “Right Honourable”.