In some respects this is answering a negative rather than a positive, because it is essentially "why DIDN'T rugby become similarly popular in Germany or Spain".

Rugby (pre-split) was played fairly widely around Europe, for instance Hannover 96 & Vfb Stuttgart in Germany & Servette in Switzerland were initially rugby sides, but no other country has a deep rugby culture like France's.

Rugby was seeded into France at multiple points principally through traders and ex-pats. Le Harve formed as a dual code club (association & rugby football) by a British railway manger in 1872 & Bordeaux had a club formed by a British wine merchant & those in the textiles trade in the 1870s too.

Pierre Coubertin was a key figure, mainly remembered today for founding the Olympics, he also helped introduce rugby to France through his book L'education Angleterre which focused on the positive aspects of Rugby School under its famous headmaster Tom Arnold. Following France's military defeat to Prussia in 1871 & the Paris Commune there was a serious crisis of culture, what had lead to this humiliating defeat and subsequent uprising and what could we do to stop it in the future?

Coubertin looked to Britain and the role sports played in its culture. Many lycees took to the game and former pupils of Condorcet & Monge schools formed Racing club in 1882, while former pupils of Saint-Louis formed Stade Francais. Both of these are still prominent clubs and recent French champions today.

France didn't have the hang ups on competition like the British did and thus formed a Club Championship in 1892 (first competed for by Stade & Racing). This helped spread the game and by 1914 the titles are spread around the areas the game is strong today, Bordeaux, Toulouse, Bayonne, Lyon, Perpignan.

From the British newspaper archive we can see that in the Edwardian era the Leicester Alliance (local select side, not the main Tigers) toured France but also Germany, while Rosslyn Park went to France but also Austria-Hungary (specifically Prague, Vienna & Budapest).

Here we get into the murkier reeds of motivations. Casual readers need to understand British class and more specifically the professional/amateur split that happened in rugby in 1895. This was a proper schism. Prior to WW1 the "northern Union" (what would become rugby league) had almost as many members as the "Rugby Football Union". The RFU was desperate to not lose any more ground to their opponents, we can see this in the handling of the 1908 professionalism allegations in the Midlands where Leicester, Coventry & Northampton were basically whitewashed of all the claims against them. The committee investigating Leicester gave their verdict on the morning of an England international held in Leicester (that was awarded after the inquiry started).

France had lost to New Zealand by only 2 tries in 1906 and regular friendlies against first England then Wales & Ireland showed they were competitive and a draw for crowds, so one reading is purely to do with skill on the field.

France also had a popular club game where players' professional careers often coincidentally seemed to result in them moving to a job just where a top team needed a player. A strict interpretation would only result in them "going north", so they were to be kept in the tent and the excesses of their shamateurism overlooked in all but the worst cases. There was no danger of Germany "going north" so they didn't need to be included.

We then get the Great War, this of course saw France as a major theatre of war and saw the mass importation of British Empire troops, there were games behind the lines between the ANZAC & British or locals, at the time Canadian football was not so different from rugby and the Canadians played too. France as an ally was then included in all the victory games, while Germany was specifically banned from playing against the home unions.

In the 1920s rugby became part of France's national culture, the Championship game was first broadcast on the radio in 1923 and it was Bayonnais rugby fans who popularised the Beret (or inspired the popularisation). This was also the decisive moment, US Quillian out right poached half the Perpignan team in 1927 and the game was burgeoning in Toulouse. "Professionalism" split the game into two competitions, the FFR with their relaxed atttude & the UFRA (Union Francais Rugby Amateur) which wanted, sort of, an amateur game. Perpignan (in the UFRA) said "in order to keep our players amateur we have to pay them twice as much").

In 1930 France came within 1 match of an historic Five Nations Championship and a crowd of 50,000 came to watch them play Wales. But the 11-0 loss, with two tries (then worth 3 points + conversion) agitated the crowd so much the referee kept play going until it was naturally near the tunnel so as to safely escape the crowd. In 1931 they were expelled until they enforced the amateur regulations & abolished their championship.

France had formed "FIRA" (Federation International Rugby Amateur), but Germany was a key member and the organisations official language was German. In 1928 Germany beat France and in 1938 20,000 watched them play in Frankfurt where Germany won again. Other than to France, Germany lost only 2 internationals in the 30s. In both world wars lots of rugby internationals lost their lives but Germany was particularly hard hit in WW2 losing 16 internationals, the most of any nation. FIRA helped spread the game in Italy, Romania and elsewhere in Europe.

France didn't return to the Five Nations Championship until after WW2, which as many can imagine changed things somewhat again. French rugby in the second world war is a whole different kettle of fish (the Vichy government regarded Rugby union & league as the same game, so when they banned all professional sport and forced them amateur they rolled the league game wholly into Union).

There is a suggestion that the IRB was less than keen on FIRA and again invited France back in to try and (succesfully) decapitate the organisation. France did not actually join the IRB until 1978.

Source: Tony Collins The Oval World + assorted rugby history books I've leafed through from my bookshelves!!