If you’re planning for a run, you want to buy up other domain names similar to the one you really want to use to prevent scumbags from creating bogus attack sites to be used to fool low information voters (not hard to do).
For example, if you want to use “walker2016”, you also want to glom onto “walkerforpresident”, “walkerforpresident2016”, and so forth.
Unfortunately, you can’t get them all.
Actually, you couldn't come close to getting all but a small handful of the most obvious names.
I think it is really a stretch to call his failure to get this particular name "poor planning".