Modern desktops

The reason why Apple didn't take off years ago was that they didn't have anything to sell to the general public. Apple computers were generally perceived to be comparatively expensive and owning one basically marked you as a graphic designer or PR weenie. That game changed when Apple introduced the iPod and, later, the iPhone; these were still expensive within their respective fields, but they were lifestyle items rather than (dorky) PCs, and helped convince people to take a second look at Apple's PC offerings, too.

The main difference between Canonical and Apple is that, unlike Canonical, Apple is a hardware company. Apple is in business to sell computers (and other gadgets), and MacOS exists because these computers need an operating system that is not Windows (otherwise, why buy a Mac?). People tend to purchase Apple's offerings – at a premium – not because of their overwhelming technical excellence but because of the slick overall impression they give. Since the hardware and OS are produced by the same manufacturer, they work well together, and Apple's customers are willing to pay extra for the comparative lack of hassle and the »coolness« they derive from walking around with a phone that has a prominent Apple logo on the back. Also unlike Canonical, Apple has been around for a while, and even people who don't actually own any Apple products generally know that the company exists and what it does.

Canonical, on the other hand, has problems getting people to pay for their product at all. In any case, it doesn't come with a computer attached, and while installing it on the computer that you already have is no longer »rocket science«, it is still a big step for most Windows users, and there can be snags (or the fear of running into snags) that spoil people's fun and act as a strong disincentive against switching over.

If Canonical really wanted to make an (Apple-shaped) dent into the Windows market, they would have to sell computers with Ubuntu pre-installed and working – and of course not just computers, but computers where already the case looked different from the run-of-the-mill PC, just to make it obvious that something else was happening. Even then it would be an uphill struggle – today, if people see you using a Mac, they think you're trendy; if they see you using an Ubuntu box, they think you're a geek.

But it's not clear to me why do you think it's impossible for Ubuntu to get it

Apple is a massive consumer electronics company. It sells huge quantities of cool hardware and music. Mac OS X is more-or-less incidental to the hardware sales; Apple has an integrated suite of products that reinforce one another in a "virtuous circle".

Canonical has... Ubuntu, one of dozens of Linux distros. (A popular one to be sure, but nothing that special.)

Because the required ingredient of the "consumer desktop" is plenty of ISVs and plenty of ISVs need uniform platform.

That will never happen in Linux. Its users are simply too ornery and opinionated (and that's a good thing, btw.) If you want to be a Linux ISV as my company is, you have to deal with that fact. It's not the end of the world. (I don't know how old you are, but I've been following the UNIX wars since the early 1980's. You should learn from history that UNIX/Linux users tend to be adamant about what they like and unwilling to be herded along by software developers.)

I don't see the need to introduce third major flavor of Linux (in addition to KDE4 and GNOME3

KDE and GNOME are not "flavors" of Linux. They're basically overgrown window managers. I use neither KDE nor GNOME, yet KDE and GNOME applications run just fine on my desktop (XFCE4). It's not like you need One True Desktop to rule the world to run Linux.

AAPL $309.92B (2 of 4683) 
     around 37k employees
MSFT $215.17B (5 of 4683)
     > 100k employees
ADBE $16.98B  (288 of 4683, 48 of 839 in Application Software)
     >9k employees
Canonical (?)
     320 employees