I think the different political system plays a role.
USA - Powerful President and two political parties in Congress
UK - Parliamentary system but the election system favours Labour and Tories.
Germany - Parliamentary system with 5 parties

The system in Germany will almost always lead to a federal coalition government. Which will be pulled to the political center. If not of it´s own free will then after it looses a few state elections. (Lot´s of federal laws need the agreement of a majority of states.)

So we don´t have - at least till now - such large swings between "right" and "left" governments. Swings in the sense of large changes in domestic policies.
Plus, because you don´t know who´ll be your coalition partner after an election it wouldn´t be wise to start smear campaigns.

So the whole situation isn´t as polarized as in the USA for example. No situation like Supreme Court vs. Gore 2000. No Iraq war. And no blatant politicization of federal agencies in Germany. No smear campaigns.

So in the USA blogs took off in the 2000s.
You either defended the Republicans or you attacked them. No other possibility left for citizens.

In Germany left-wing people dissatisfied with Schroeder founded a new party. Maybe that´s a reason too. New parties have it much easier in Germany than in the USA or UK. Green Party in the 1980s, "Die Linke" now in the 2000s.
People who might become bloggers (in the USA) can help grow a new party here (in Germany).