There are limitations to how much can be done on a phone. On the other hand, tablets are considered to be computer replacements and ppl expect alot more. There is also more things they can do with bigger screen..better processing ability..etc.
The iPhone helped reshape the smart phone industry but then again there are many things ppl are not willing to pay for and thus a broader range of phones is necessary with different features, omissions and price points.iPhone doesnt cater to all those categories as of now.
On the other hand..the tablets face competition within the same segment and from new tech laptops..desktops and other computing devices, thus it needs to be upgraded more more frequently than mobiles.
Although personally speaking..apple is trying to slightly up the tempo this year with slightly earlier launch dates compared to 2012.