Hugo Brady outlines some of the reasons:
The balance of the parties matters for the leadership of the European commission. In June 2009, the European council is due to nominate the commission’s next president. European Union leaders are likely to offer José Manuel Barroso, who is affiliated with the EPP, a second five-year term. But if the PES becomes the largest group in the parliament, they will try and insist on one of their own. The newly elected parliament is due to approve the European council’s nominee for commission president in July. If the centre-right does end up dominating the parliament, Barroso will be voted in.
In autumn 2009 the parliament will hold hearings on the individual commissioners proposed by governments. These hearings matter. In 2004, the parliament did not like the look of Silvio Berlusconi‘s nominee, Rocco Buttiglione, on account of his views on gays and women – and it forced Berlusconi to withdraw him.
In January 2010 the parliament will vote to invest the entire team of commissioners. If it is implemented, the Lisbon treaty will make more explicit the need for the appointment of the commission president to “take into account” the results of the European elections. In the long run, whatever happens to that treaty, the commission is likely to become more directly accountable to the parliament. But whether that makes Europeans any more willing to vote for MEPs is another matter.