Consider the contrary: what do you get with a thicker computer? These iMacs already have performance (top-end: 3.4 GHz i7, 32 GB RAM, GTX 680MX with 2 GB GDDR5, thunderbolt and USB 3). If they said, "okay, instead of making it thinner, let's keep it the same size and make it better," what exactly do you expect?
The only improvements you could get with a thicker computer are things that would also negatively affect the amount of power it requires. Thinness is just a pleasant consequence of power-concious design.
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Consider the contrary: what do you get with a thicker computer? These iMacs already have performance (top-end: 3.4 GHz i7, 32 GB RAM, GTX 680MX with 2 GB GDDR5, thunderbolt and USB 3). If they said, "okay, instead of making it thinner, let's keep it the same size and make it better," what exactly do you expect?
The only improvements you could get with a thicker computer are things that would also negatively affect the amount of power it requires. Thinness is just a pleasant consequence of power-concious design.