Frankfurt is too big to fail as a German capital.
Frankfurt was always the mercantile crossroads of Germany and central Europe.
Frankfurt was always the "neutral" ground between all the little Germanies, it used to be a free Imperial City before 1866.
Last but not least Frankfurt was the HQ of the U.S. occupation troops in Germany.
It's also been the HQ of the combined Bizonal economic council where the respective Länder from the zones convened.
If Frankfurt had been chosen as West German capital, it's very likely that it remained the all-German capital today.
Most of these things are logical consequences rather than new reasons. It's actually a great map contest idea to show off how the German government quarter in Frankfurt would've looked like. Bonus points if you're considerate enough to integrate what eventually became the HQ of the Hessischer Rundfunk (Broadcast of Hesse): It was initally built to house the new Bundestag and repurposed when it was clear that Bonn got the job.
But without the defeat and division of Germany, once Prussia succesfully acted up in 1866, Berlin was set in stone to become and stay the German capital. Only the small window of opportunity after 1945 could've changed that. Otherwise, Berlin already was too big to fail as a German capital.
Frankfurt looks as if it was supposed to stay, but the big problem is that Berlin was just there and once reunification happened, people would've thought about how to put Berlin into good use even if Frankfurt instead of Bonn was the factual German capital. And here we go again with a return to Berlin.
I recently read that the envisioned equal division of labor between Bonn and Berlin is already skewed into direction Berlin, saying that 62% of all federal government jobs already are in Berlin. Bonn was actually envisioned to house the majority of those jobs or at least half of them. Now it looks like public opinion MAY actually be in favor of abandoning Boon completely. As I said, MAY.
PS: OTL is already pluricentic to a degree even in West Germany. Federal Constitutional Court always was in Karlsruhe, some other courts have be relegated back to Leipzig that housed the Imperial High Court back in the days etc.