Association football and, at least in France and Belgium, cycling are popular and well-established at a grass-roots level. Baseball will need to connect with locals to have any longevity in Europe.
Arguably, MLB really missed its opportunity during the height of the Cold War when hundreds of thousands of Americans were stationed in Europe. They could have had barnstorming tours by players of U.S. bases, with servicemen bringing in local kids to watch and building a long-term fan-base. And now, not only are most of the servicemen gone, but so is the infrastructure. The former Leighton Barracks in Würzburg, for example, had at least four baseball fields; all are gone now.