I think there are a few reasons the top 5 teams are monopolising the points so far.
1) Red Bull, Mclaren, Ferrari, Merc and Renault all have had bullet proof reliability during races and none of their drivers have crashed out yet.
2) With DRS, KERS, and pirreli tyres its relatively easy to pass compared to last year so the top 5 teams are less likely to get stuck behind a mid field car and ruin their strategy. If they do its not for long as theres a pit stop around the corner and a chance of some clean air anyway.
3) We havent seen any rain or safety car periods yet, both of which tend to bring the mid field closer to the front.
4) I think this season Mercedes and Renault have jumped from being at the front end of the midfield to being regularly either with or just behind Red Bull, Mclaren and Ferrari, which means fewer opportunities for the teams behind to pass them.