Channel nine is rubbish, I'd agree with that, but I don't think that is why Top Gear is failing in the ratings.
Watching Top Gear on nine is not all fail. Yes, the ads are more intrusive than they were on SBS, but nine are at least showing full, un-cut episodes, which I don't think SBS did.
The problem(s) as I see it are:
1. Saturation and repeats. Since they stupidly spent so much on gaining the rights ($20 million), they obviously feel they need to use them (saturation) and unfortunately for nine, the powers-that-be at Top Gear HQ can't be bothered making more than a handful of episodes a year. Last year, there were only 8 new episodes (including the America 3 and Wise Men specials) .. that is barely 2 months of new programing a year. Repeats are inevitable if they are to get any return on their investment.
2. The show is dieing. Instead of being inventive and informative, it has become formulaic. You can be pretty sure when you tune in now (to a new episode) you will see one of them testing a car that 0.0001% of the population can afford on an airfield and judging it on how well it can be driven in a fashion that limits the lifespan of the tyres to a couple of miles; an interminably long interview with a guest where Clarkson strives to turn the discussion away from cars; some news; and an excessively expensive challenge where a lot of their budget (which could have been used to make more episodes) is blown on an attempt to emulate Frank Spencer in "Some Mothers Do 'Ave 'Em".
3. Top Gear Australia. This is even worse than current Top Gear and lurks like that annoying little brother, with a handful of your baby photos, ready to jump in and spoil your 14th birthday party. A lot of the decline in the ratings has occurred after the airing of (the reprievingly short) 3rd series. I wouldn't mind betting that some of this decline is the result of people not willing to risk tuning into Top Gear, only to find they have committed to watching Top Gear Australia instead.