Inspired by a link in Ken Graham's answer I read the Wikipedia article on Archbishops. That contains a paragraph germane to the question but which Ken doesn't quote:
Roman Catholic bishops and archbishops are styled "The Most Reverend" and addressed as "Your Excellency." In English-speaking countries, a Catholic archbishop is addressed as "Your Grace", while a Catholic bishop is addressed as "Your Lordship". Before December 12, 1930, the title "Most Reverend" was only for archbishops, while bishops were styled as "Right Reverend." 
- Canon Law Digest, Bouscaren, Vol. 1, Page 20. Rt. Rev. Dominic Laurence Graessel. Roman Catholic Archdiocese of Baltimore. Retrieved on 2016-11-19.
The article on Graessel says
Before December 31, 1930, the title "Most Reverend" was reserved for archbishops; the other bishops were given the title of "Right Reverend." After that date, "Most Reverend" became general for all archbishops and bishops. (Canon Law Digest. Bouscaren, Vol. 1, P. 20).
Thus there are two potential dates for the introduction of "Most Reverend" for bishops.
I then searched for "31 December 1930" decree "Most Reverend" and found the Wikipedia article on the honorific Excellency which gives further details:
By a decree of the Sacred Congregation of Ceremonial of 31 December 1930 the Holy See granted bishops of the Roman Catholic Church the title of Most Reverend Excellency (Latin, Excellentia Reverendissima). In the years following the First World War, the ambassadorial title of Excellency, previously given to nuncios, had already begun to be used by other Catholic bishops. The adjective Most Reverend was intended to distinguish the religious title from that of Excellency given to civil officials. ...
In some English-speaking countries, the honorific of Excellency does not apply to bishops other than the nuncio. In English law, Anglican archbishops are granted the title of Grace (Your Grace, His Grace, as for a duke), and bishops are granted the title of Lordship. The same titles are extended by courtesy to their Catholic counterparts, and continue in use in most countries that are or have been members of the Commonwealth. An exception is former British East Africa (Kenya, Uganda, Tanzania).
- Acta Apostolicae Sedis 1931, p. 22; L'Osservatore Romano 24 January 1931.
The Acta are available online, although only in OCR form and thus do contain reading errors. The relevant decree is from what is usually called the Sacred Congregation of Rites and was indeed published on 31 December 1930:
SACRA CONGREGATIO CAEREMONIALIS
DE TITULO « EXCELLENTIAE REVERENDISSIMAE »
Ssmus D. N. Pius Papa XI , eo consilio ut dignitas eorum, qui cum in
Ecclesiae gubernatione, tum in ipsa Pontificis Maximi Domo principes
habent partes, maiore cotidie in honore sit, Purpuratorum Patrum percontatus
sententiam, qui sacris caerimoniis regundis praepositi sunt, die
11 mensis Decembris huius anni, arcessito viro hac in pagina subsignato,
qui Sacrae Congregationi Caerimoniarum est a secretis, decrevit: Excellentiae
Reverendissimae titulum, praeter quam Patriarchis et Latinae et
Orientalis Ecclesiae, praeter quam Praelatis qui a flocculis vulgo appellantur,
praeter quam Nuntiis et Internuntiis Apostolicis, tribuendum
quoque esse Archiepiscopis atque Episcopis sive residentialibus sive titularibus
tantum, itemque Magistro Pontificii cubiculi, Praelatis qui assident
vel sunt a secretis in Sacris Romanis Congregationibus, Secretario
Supremi Tribunalis Signaturae Apostolicae, Praelato Decano Sacrae
Romanae Rotae ac denique Substituto Secretariae Status. Contrariis
quibuslibet non obstantibus.
Datum Romae, ex aedibus Sacrae Congregationis Caerimonialis, die
31 Decembris 1930.
gg-I. Card. GRANITO PIGNATELLI D I BELMONTE,
Ep. Ostiensis et Albanensis, Praefectus
B. Nardone, Secretarius.
THE SACRED CONGREGATION FOR CEREMONIES
DECREE ON THE TITLE "MOST REVEREND EXCELLENCY"
So that the dignity of those who have the primary part of the governance of the Church, and especially of those in the house of the Pope itself, Our Most Holy Lord Pope Pius XI, after inquiring the opinion of the Purple Fathers who have been placed in charge of the rules for sacred ceremonies [i.e. the members of the Sacred Congregation for Ceremonies], has decreed by this advice, given on the 11th of December of this year, and undersigned on this page by the representative called for from those set apart for the Sacred Congregation of Ceremonies:
The title of "Most Reverend Excellency" (except for Patriarchs of the Latin and Oriental Churches, and for Prelates who are given that name by courtesy, and except for Apostolic Nuntios and Messengers), is to be given to Archbishops and Bishops, both [bishops] by residence and [bishops] by title, as well as to the Master of the Papal Household, and to Prelates who assist at or are members of the Sacred Roman Congregations, the Secretariat of the Supreme Tribunal of the Apostolic Signatura, the Prelate Dean of the Roman Roat, and finally the Substitute Secretary of State. Anything at all to the contrary notwithstanding.
Given at Rome, in the offices of the Sacred Congregation of Ceremonies, on the 31 December 1930.
gg-I [sic] Cardinal GRANITO PIGNATELLI DI BELMONTE, Bishop of Ostia and Albano, Prefect
B. Nardone, Secretary
(translation Matt Gutting)
This says that the title "Most Reverend" tribuendum quoque esse Archiepiscopis atque Episcopis sive residentialibus sive titularibus tantum, "shall be given also to archbishops and bishops who are residential and also titular."
Thus my first hypothesis is correct and prior to this decree bishops were Right Reverend, as they still are in the British Isles and most Commonwealth countries.
The question still remains as to why the decree was not taken up within the Catholic hierarchy in Britain and the Commonwealth, although the cryptic reference to "English law" in the Excellency article is probably the reason. If I find that before someone else does, I'll update this answer.