Apple stuff is all about the ecosystem/integration into your life now, and they wouldn't want to alienate a large % of users by changing too much in one go and making a load of things exclusive to the newest product. The reason they have such huge brand loyalty is because they know how to keep customer satisfaction super high. Also unlike phones, which are mostly sold on contract and get updated every 18-24months, you get the impression, like the Macs, and iPods, that people won't be upgrading as quickly with this (although probably not quite as slowly as the after mentioned lines), it's more an investment. It's not like Apple are the only company that do this, everyone does it (except with poorer legacy support lulz), and although I don't really count the days, ASUS and Samsung seem to churn out new product revisions waaaay more quickly. At least with Apple you have a fairly consistent schedule.
The iPod is no longer wow, the iPhone is no longer wow and the next one won't be either, the iPad is starting to lose it's wow. Macs have lost their wow ages ago. Once you've created a product that's got a very defined role/use, unless you change it radically and it becomes almost something else, then you're just improving on it and keeping it up to date, it isn't 'woah' any more. Not a bad thing, people still buy it because they know it's good and they know what it is.
I'm sure Apple understand that completely: while everyone is jumping on the ultrabook and tablet wagon, I'd wager their biggest creative types have their teeth sunk firmly into new product lines, obviously the proper Apple TV being one - and that is definitely something people won't be changing every year (unless you're very rich and slightly mental) - which is where the next 'wow' will come from.
Also humans, short attention spans etc. And yes, when you're held to high standards like Apple you want 'wow' all the time when it's not realistic.