Why (in broad, speculative terms) have MLB teams relocated?

This will follow up the previous post that was looking at MLB team relocations/expansions. I'm ostensibly doing it for my OOTP solo league, but apparently I'm on a roll. There will probably be a third in this series, looking at the specifics of how that league will evolve given what I've been thinking in these posts... All of this is basically unreferenced at this point, some of it is speculative, and some of it is based on memory. I make no claim to be a historian, and I'm not looking for peer review. :)

Why did the Athletics, the Braves, the Dodgers, the Giants, et al. relocate? And why was there a big rush of relocations in the 50's/60's after a half-century of nothing?  Here, I think it's a function of the technology available and the nature of fandom at a given time, plus I suppose an evolving relationship between teams and their homes.

In 1950, five cities had multiple MLB teams. In 1960 only Chicago did. In 1950, four of the 13 largest U.S. cities were without an MLB team. In 1960 all 13 had a team. None of the moves in the 1950s left a city empty of MLB baseball. Several of the moves in the 1960s and after left cities empty--Milwaukee was empty 1965-1969, Washington was empty from 1973-2004 (if I remember right), and Seattle was empty 1970-1976.  Montreal's been empty since 2005. Several other city-emptying moves have been contemplated but not quite completed, including Minnesota and Tampa Bay to various spots. 

I think that first wave of moves had something of a different character than subsequent ones, presumably because it became impossible (or seen as impossible anyhow) for any but the very largest U.S. cities to host more than one team. I think that the Dodgers/Giants move was a harbinger of (many/most of) the more recent moves--teams moving not because they couldn't survive elsewhere but because they weren't making as much money as they'd like. But maybe more on that later?

There were a pair of relocations in the early American League, but those seem to fit the earlier discussion of "underserved places/underpaid players" more than modern-day relocations--the Milwaukee franchise moved to St. Louis and the Baltimore franchise moved to New York in order to put additional pressure on the National League. There was a Federal League relocation (Indianapolis to Newark) that had similar motivation. The first relocation discussion that I can find that is more of a modern one was the aborted move of the St. Louis Browns to Los Angeles for the 1942 season. So, by 1940 or so it became clear to at least some observers that relocation (even to the West Coast!) was an option.  

I think that the rise of radio and how it affected fandom led to the end of (most) multi-team cities, and TV hastened it. In 1920, teams could (and did) share ballparks. The only ways to be informed of baseball results in real time was to either be at a game in person or maybe go to one of the setups where people were connected by telegraph and updated an early version of Gameday. So, the Browns could let the Cardinals use Sportsman's Park because they weren't going to make any money from it that day, anyhow. And with average attendance at or below 10,000 (or even 5,000) per game, there was plenty of room for different teams in the cities (as long as they were cooperating on schedules and the like). 

Through the 1920s and 1930s, radio came in and provided additional revenue streams. Now, baseball fans in St. Louis had the choice of going to the park and watching the Browns or staying home and listening to the Cardinals (or vice versa). And folks out in rural Missouri (and as far as the signals would carry) could choose between listening to the Browns or the Cardinals on the radio (assuming the Browns games got broadcast at all). Given that the Cardinals were riding a string of pennants with Hall of Fame players and the Browns weren't, it isn't hard to imagine that the extra ad revenue from radio broadcasts helped the Cards cement their position as top dog in St. Louis by letting them invest in minor league teams and better players. I'm speculating here, but it seems to make sense. As far as other cities, I think the balance of power between teams was less obvious--the Athletics were much better than the Phillies in the very early 30s, but both were terrible and drawing terribly until just after WWII. I have read elsewhere that it was effectively luck of the draw that kept the Phillies in Philadelphia--both teams got attendance boosts for the first couple of postwar years, then the Phillies got good and the A's got worse and the attendance for the latter cratered. Boston seems to have become a Red Sox town starting in the mid-30s, and the Braves never outdrew them again even when winning the pennant in 1948. 

Once the bonds were broken between the teams that moved and their original homes, I think there was pretty clearly a period of much greater flux for them. Of the teams had not moved as of 1960, only the Washington Senators would ever move in the future (and they arguably found themselves put into a 2-team city given the proximity of Baltimore and DC, and they were immediately replaced via expansion).  So, while there were occasional attempts to woo Cleveland, Pittsburgh, the White Sox, or others, they clearly all found themselves insufficiently motivated to move either because they owned their cities, didn't have better alternatives, were afraid of lawsuits, or were content with their markets.