Possibly because Cameron has been very definitive in his handling on the sleaze with the MEP's and such. He's sent in the "Cameron-Cleanup Crew" and is likely to order the replacement of all the Tory MEP's involved in the corruption.

That and the British people are becoming increasingly euro-sceptic and have been told repeatedly by MEP's from both the Lib Dems and Tories that it's a horrible bubble that actively encourages such conduct and that hasn't been audited properly in the last 15 years also helps the Tories more than hinders them. Cameron's coming up roses by being definitive and rapid in dealing with this. In a single week he's forced or sacked the worst offenders, and fired his cleanup crew to make them more accountable.

Labour also remaind eeirly silent on the whole sleaze issue over the weekend also, with only the Lib Dem's  who's MEP, iirc blew the whole report in the first place, being the loudest about it.

In my opinion, however, I feel that most members of the public are aware that all politicians are sleazy in one way or another, and at the moment there's only one party repeatedly calling for calls of transparancy, and is actively pursuing it.

It's MP's are now on the voluntary declarations-of-expenses register in Parliament, and now it's MEPs are getting the crap kicked out of them for claiming money and funneling it when they shouldn't.

It's perhaps Cameron's decisive moves, and even ensuring his house is in order long before a GE that's keeping the momentum with him. 

.... that no-one gives a d*mn about MEPs. Europe is seen as one big gravy train.

The 10 year old allegations against Spelman have more weight than yesterday's allegations against MEPs and even the Spelman allegations look very weak.

Finally there is the curious "british electorate" effect. Scandals that damage the parties in the eyes of the electorate have, historically, depended on the party. Tories are damaged by sex scandals and Labour by money scandals. On that basis the tories are safe for now....

Even given the public non-interest in the recent Tory sleaze allegations (which, as people have alluded to above, are quite inconsequential, especially given that the Spellman allegations are 10 years old), our own party's flirtation with sleazy activities in recent years will hardly see the electorate turn back to us as the 'non-corrupt' alternative. After 11 years in office, we have accumulated as bad a reputation as the Tories (perhaps unjustly so, but that is what I fear 'public perception' to be at the moment), so any current or future allegations of corruption against them will not benefit us, but increase the levels of cynicism and apathy amongst the electorate.

It is unlikely that the Tories will do anything wrong that between now and the GE that has enough gravitas to swing the support our way.

Our opportunity to swing the polls back to Labour will either come from when the Tories finally put policy to paper and (hopefully) reveal their true colours or (my preference) take control of the agenda ourselves and make some positive and progressive strides in policy over the coming months which will capture the imagination of the electorate.