Since 1998, MLB has had almost an identical composition as it does today. The Expos moved to D.C. and became the Nationals in 2005 and the Astros moved to the American League in 2013, but the greatest structural change since ‘98 has been the Marlins rebranding from “Florida” to “Miami”.
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The lack of expansion has been attributable to any of a number of factors. Owners split the pie on many of MLB’s revenue streams, so slicing that more thinly isn’t popular, and would not necessarily be immediately mitigated by additional revenue from the new club(s).