I think the best report I have read online in regards to this was from The Verge. Chris Ziegler did a great job explaining why (in a way) Android seems to be dominating the market.
(This photo does an excellent job at demonstrating the point)
Image
http://www.theverge.com/2011/12/23/2651347/a-good-smartphone-comes-but-once-a-year
To further elaborate on
mbell1975 point, the iPod never really had competition until the Zune, but by then it was too late. The iPad is still in its early phase (just like the iPhone was in 2007-2009), but competitors can always catch up if you make a reasonably good product at a reasonable price, or if you saturate the market with multiple handsets in a variety of form-factors.
Many competitors were caught off guard by the success of the iPod and never thought that a computer company can build a phone (despite all the rumors) and actually be successful (twice in a row), so when the iPhone was announced many of them dismissed it thinking that what they had to offer would continue to dominate the market.
Microsoft had attempted for several years to put a desktop OS on a tablet and was unsuccessful, so when rumors began to circulate that Apple was going to build a tablet and rumors suggested that it would cost around $999 (because Apple is known as a premium company), once again manufacturers dismissed it thinking it would not succeed. But Apple getting that starting price point at $499 was astonishing and a big factor to its success.
Now I am seeing a repeat occurring with just rumors of Apple one day releasing an actual TV.