Putting an "incomplete" park in
Disneyland's parking lot is a completely different proposition than making Animal Kingdom as an incomplete park.
First of all, DCA is only the second park on the site. The main reason for building the second park was to put Disneyland on the map as a destination Resort, with extended stays on site. This was an attempt to emulate the success of WDW.
It was the opening of EPCOT that really catapulted WDW into the "destination" category, with week and longer stays. This was made possible by creating EPCOT as a 1.5 to 2 day park, and even at opening it mostly lived up to this billing. I know many people are somewhat down on EPCOT, but without it the huge build-out of all the resort hotels never would have happened (at least not at nearly the pace we witnessed over the last 15+ years). Compare this to poor DCA, where you can do most everything in 6-7 hours (half a day, regardless of the number of hours the park is actually open), and which requires much less time than that on repeat visits to do the very few things that aren't in the been-there-done-that category. Not what it takes to make DL a destination resort. If AK (or MGM) is incomplete, well that's okay, because WDW was already a destination resort.
DCA also has the additional disadvantages of being in Disneyland's parking lot. It is so close to DL that it invites direct comparisons in ways totally unlike the widely scattered parks at WDW. When at DCA, DL is right there in-your-face. You can't escape the feeling that you should be across the way at DL instead of here at DCA.
And for the most part (until this recent announcement), you can't park hop because this was only available to APers and those with many-day tickets which expire (and as already pointed out, you don't need tickets for that many days at the DL resort). I don't mind if AK is not a full day park, as I am there on a park hopper and just hop to another park when I am done. I would never pay a full admission to AK, but that's okay because I don't have to. I had to pay a full admission to see DCA, but I would never do so again.
And DCA is not adjacent to just any park, it is adjacent to Disneyland, the flagship park which has the most attractions of any Disney park. Being sooo close to DL which has soooo much to do, makes DCA look even more sparse by comparison.
Finally, while it is a smaller matter, DL has a level of tradition that surpasses WDW. It has served an additional generation of guests as compared to WDW, most of whom are locals which make up the bulk of visitors and have a special fondness for the place. It is a slap in the face to these people to put in an inferior, incomplete product and charge full price for it.