Some, like Crane Song, offer it...they just make it a pain in the ass to get to. The Avalon 747 [and for that matter, the 737] have the 'side chain capabilities' right on the front panel. The Drawmer 1969 has side chain capabilities...it even has my favorite one for '2 Bus applications' on a switch on the front.
I know the Pendulum OCL-2 has a "sidechain insert" as well as internal sidechain options, the ES-8 and the 6386 also have internal sidechain options [these relate to the time contants/variables of the units rather than frequency dependant variables], I don't recall if the ES-8 or 6386 have a sidechain insert point or not.
I would reckon that the reason that alot of the 'upper' level people don't do it is cost. People (as in consumers) already piss and moan about the price of the units, so those folks do what they can to keep the quality high, and the price as reasonable as possible.
It's not all that hard to do, in fact the most expensive parts to accomplish the task are the connectors (though if they put the connections on barrier strips it might not be that bad :p ).
The $500 boxes do it because they want to give you every possible function known to humans for the least amount of money to make up for the nearly universal fact that they couldn't pass audio any easier than a kidney stone.
I'd rather not have side chain capabilities and decent audio than a ton of a control and a cheesey sounding piece of superfluous dung [the RNC being one of the few exceptions to that rule]. But that may just be me.