I'll admit that the Celtics had an easier time in the sense of a smaller league and it was easier to stay dominant longer. They won 8 in a row at one point, and that's not going to happen with today's league. Hell, 3 in a row these days like the Bulls and Lakers did is probably as hard as it was to win 8 in a row those days. But, the Lakers won their five in Minneapolis in a span of 6 years for the same reason - one player could make all the difference in the league back then, in the Lakers case George Mikan, in the Celtics case Bill Russell.
But all of that's moot to a large degree. The same situation of expansion exists in all leagues, but that's no excuse to belittle the accomplishments of earlier eras. The Celtics still have scoreboard with 16 championships, and until the Lakers (or less likely, but still possible, someone else) gets at least as many, the Celtics get the bragging rights dynasty wise.