Yield, warranty, and overclocking factor.
Much easier to get chips at 3.6ghz than 4.5ghz, and to maintain a warranty on those chips will cost much more if Intel sold them at 4.5ghz.
Plus, enthusiasts who buy these chips will want to OC them anyways. We'd rather buy a 3.6ghz and OC it to 5ghz than buy a 4.5ghz chip and "only" be able to OC it another 500mhz.
it's a TDP limit.