One reason is the business cycle. The 2000 Census was taken on April 1, 2000, less than a month after the peak of the tech boom. Unemployment was low, immigration was high, and entry-point houses and apartments were crammed with large families.
The 2010 Census was taken after two years of recession, when immigration had slackened off. We simply don't know whether this was just a temporary response to the business cycle or the beginning of a permanent decline in migration.
Past mass migrations, which most experts expected to continue indefinitely, in fact ended abruptly. Net Puerto Rican migration to New York City stopped in 1961, and the huge movement of Southern blacks to Northern cities ended in 1965. Those who extrapolate current trends far into the future end up being wrong sooner or later.
Finally there is an assumption -- which is particularly strong among those who expect a majority "people of color" electorate to put Democrats in power permanently -- that racial consciousness never changes. But sometimes it does.
American blacks do have common roots in slavery and segregation. But African immigrants don't share that heritage, and Hispanics come from many different countries and cultures (there are big regional differences just within Mexico). The Asian category includes anyone from Japan to Lebanon and in between.
Under the definitions in use in the America of a century ago, when Southern and Eastern European immigrants were not regarded as white, the United States became a majority non-white nation sometime in the 1950s. By today's definitions, we'll become majority non-white a few decades hence.
But that may not make for the vast cultural and political change some predict. Not if we assimilate newcomers, and if our two political parties adapt, as we and they have done in the past.