Why are church reformist centers all outside the boundary of the original Roman Empire?

the church reformist centers are virtually all outside the original Roman Empire

That is not really true.

The oldest reformist or heretic movements appeared within former Roman territories, like the Bogumils - in the Balkans - and the Cathars - related or not to the previous, in south of France, which later, during the religious wars of the 16th century, was a protestant stronghold .

Those were movements that affected large populations and territories, but many innovative and less orthodox lines of thought had less to do with populations and territories than with theological, philosophical and scientific studies and controversies, related to large urban and intellectual centers in France and especially Italy.

Joachim of Fiore, Abelard, Francis of Assisi are among the most renowned of the many such controversial/reformist thinkers. Many others anticipated future reforms and should be considered, from Amaury de Chartres, Gerard Segarelli and Fra Dolcino to Savonarola and Giordano Bruno.

The success of anti-Catholic reform in Germany and England can be more easily explained by reasons related to geography (as a periphery of Catholic world in relation to the center of Rome and Italy) and especially to stages of political and economic development and innovation in the context of changing relations between king and Pope, between religious and non-religious life and powers, increasing local autonomy, economic, technological and institutional mutations etc.

Luther and Calvin were Christian intellectuals (theologians, philosophers) and should be considered within that framework, instead of a rudimentary and simplistic scheme about the borders of the Roman Empire.

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Of course, there is an intellectual polemical tradition, both Catholic and Protestant, of encouraging precisely this opposition between a Protestant anti-Roman and Germanic "north" and the Catholic, Latin, "south", one that was transformed in recent times by the latest ideological oppositions involving fascism, Nazism, capitalism and communism. Some Catholic thinkers have connected Protestantism and Nazism with a supposedly reduced presence of "real" Christianity in German lands. Others have connected/related Protestantism with democracy against Fascism and Catholicism. Some have connected (equated) Protestantism with capitalism (some as a bad thing, some as a good one; not to mention the infamous Judaism <--> Capitalism). Others have related Communism and even Stalinism to Eastern Christian Orthodoxy. (By the way, the Tsarist autocracy is descending straight from the Byzantine tradition, that is from the Roman imperial one.) But it seems obvious to me that all such connections are very dubious, hazardous, and essentially ideological and polemic.

(It has always struck me the Anglo-Saxon "popular" trend, for example on Discovery Channel-style TV history shows, of presenting Catholic countries, especially Spain and Italy as bulwarks of Inquisition-style oppression and backwardness in contrast to the open-minded liberty-prone England. — In such shows everybody expects the Spanish Inquisition, and it never fails to appear — hence the Monty Python joke I guess.)

Is it a coincidence that the church reformist centers are virtually all outside the original Roman Empire border, the Roman Limes ?

No. It is not. It is by no means some sort of mere coincidence that Roman Christianity persisted within the confines of Romance civilizations, whereas Protestant Christianity blossomed primarily in Germanic areas. A closer look at persisting schisms will quickly reveal cultural and religious boundaries to overlap quite often; thus, we have Nestorianism in the Middle East, Monophysitism in North Africa, Eastern Orthodoxy in Eastern Europe and Roman Catholicism in Western Europe. Most Germanic tribes first converted to Arianism, before eventually embracing Imperial Orthodoxy. Oftentimes, in his (polemical) writings, Martin Luther conceives of the Roman Church and its Sovereign Pontiff as a foreign power and alien despot, tyrannically oppressing the Germans, either linguistically (by holding services only in Latin, and recognizing Jerome's Vulgate as its sole official Bible), or financially (by church taxes and the sell of indulgences, all so that an imposing religious palace might build in their own country).

Consider the Church of England as a counterexample, or for that matter Calvin in Geneva. If you look for a simplistic explanation, look not at Roman borders but at the political weakness of the Holy Roman Empire and cuius regio, eius religio.

There were various reform (or schismatic) movements, like the Cathars or the Lollards which are little known because they got crushed. For that matter, the Hussites got severely suppressed as well.

Luther came in a time and place where he could find patrons. Read about survivor bias to understand why many other reformists are almost forgotten.