Game publishing is a high-risk, high-yield business. Most games lose money. The hits are expected to compensate; therefore, small publishers (who can't afford to release many titles) struggle, and big ones hedge their bets through mergers and acquisitions.
For developers, concentration of the publishing business is unwelcome: fewer outlets for games means less favorable deals. However, until online self-publishing becomes a viable option for high-budget games, the trend is likely to continue unabated.