How did the term "esquire" come to be used for lawyers?

According to Wikipedia's page for Esquire:

According to one typical definition, esquires in English law included:

• The eldest sons of knights, and their eldest sons in perpetual succession
• The eldest sons of younger sons of peers, and their eldest sons in perpetual succession (children of peers already had higher precedence)
• Esquires created by letters patent or other investiture, and their eldest sons
• Esquires by virtue of their offices, as Justices of the Peace and others who bear any office of trust under the Crown
• Esquires of knights constituted at their investiture
• Foreign noblemen
• Persons who are so styled under the Royal sign manual (officers of the Armed Forces of or above the rank of Captain in the Army or its equivalent)
• Barristers (but not Solicitors)

A slightly later source defines the term as

Esquire — A rank next below that of Knight. Besides those Esquires who are personal attendants of Knights of Orders of Knighthood, this title is held by all attendants on the person of the Sovereign, and all persons holding the Sovereign's commission being of military rank not below Captain; also, by general concession, by Barristers at Law, Masters of Arts and Bachelors of Law and Physic.

This practice presumably travelled across the pond where it was, over a period of time, abandoned by everyone except lawyers.

This topic is covered in an article in the American Bar Association's ABA Journal on the ethical ramifications of the use of the title by lawyers.

In Opinion 1995-14 (1995), the committee traced the origins of esquire to the Middle Ages, when it was a title conferred on candidates for knighthood in England. Later, the term was extended to other mid-level dignitaries, including sheriffs, sergeants, justices of the peace and “barristers at law.”

In the United States, esquire over time came to refer “commonly and exclusively” to lawyers, stated the opinion, but how that happened is a mystery. The only certainty, the committee stated, is that “based on common usage it is fair to state that if the title appears after a person’s name, that person may be presumed to be a lawyer.”

So, while lawyers were one among many professionals who were using the suffix back in Merry England, it's unknown why only lawyers continue to use it in the US.

Esquire (in BrE) means not sir- it's a default title for somebody who doesn't otherwise have a title.

I suppose some American lawyer wanted to sound important and didn't understand the irony.

SUPPLEMENTARY TO coleopterist's ANSWER:

Instructions of Sir Henry St. George, Clarenceux, to Thomas May, Chester Herald, and Gregory King, Rouge Dragon Pursuivant, and Henry Dethicke for the 1682-83 Visitation of Worcestershire.

 In the allowance of titles, you shall enter the persons whose descents you take, with no other titles but such as they may justly and lawfully bear according to the Laws of Arms. And you shall allow the title of Esqr. to these and no other :—

 1. The heir male of a the younger sons of Noblemen.
 2. The heir male of a Knight.
 3. Officiary Esq^rs., viz., such who are so made by the King by putting on a collar of SS.,
    or such who are so virtute officii, without that ceremony, as the High Sheriff of a
    County, and a Justice of the Peace, during their being in office or commission ; with this
   caution, that you always enter the said office or qualification in speciall [sic] terms.

 As for Sergeants at Law, Doctors in Divinity, and dignified Prebends, you shall register them by those titles or qualifications only, but you shall accept them in quality as an Esq^r.
 Barristers at law you shall enter by that title, but you shall accept them as gentlemen only unless otherwise qualified to bear the title of Esq^r.

SOURCE: The Heraldry of Worcestershire (1873), by H. Sydney Grazebrook, Esq.; Of the Inner Temple, Barrister-at-Law, Author of the “Heraldry of Smith,” &c., &c. I, xlviii-xlix.

Grazebrook adds that “Barristers are now always accounted esquires, their right thereto being sufficiently attested by the fact that the Court of Common Pleas refused to hear an affidavit, because a barrister named therein was not called esquire ; and now all the Courts invariably require counsel to be so styled.--See Burke’s Patrician, v., 114, and Bythewood’s Conveyancing ii., 386, where the question of “Who is an Esquire?” is discussed at length.”

I'm an attorney, admitted to the bar over 20 years ago in California, and I practice in-house (not in a law firm). Depending upon the content and intended reader of my correspondence, I may deliberately sign my letters and emails with my name, followed by "Esq." because I want it to be obvious that I am a practicing attorney, and the correspondence may be protected by the attorney-client privilege. It's intended to be an obvious signal to the immediate reader - and in any potential litigation - that the correspondence might be protected as legal advice and should be reviewed before disclosure. My two cents. JT