I think he was familiar with this area. I imagine it may have been somewhere quiet he hung out with friends while drinking or doing drugs, it's the city limits. He may have been fairly confident he could drive in there and dig a hole, perhaps concealed behind his vehicle from any likely vantage point. He likely was drug affected further suppressing normal inhibitions and fears. He may have been flying high having finally realised his violent fantasies or so panicked he had to act quick, but what he wasn't, was a cool operator, level headed. The 11 calls suggest to me he was panicked and unsure.
So many murderers of family members seem to dispose of the bodies nearby. I guess it could be simple stupidity, but I wonder what the decision process is like? I imagine a lack of resources - time, transport, equipment to conceal the body, and then fear - of being discovered enroute, of what they've done, what they have to do next, means getting the job done quickly with as many variables controlled as possible. One of those variables being having an idea of where you are putting them and what that might involve. I think a degree of laziness comes into it and so if they are the sort of person who has had their indiscretions indulged and glossed over all their lives, they may know intellectually they will be suspected but emotionally they have lots of practise being too slippery to get caught. Mediocre but successful lies all their life, mediocre cover-up.
If part of the crime was to do with sexual sadism, or that he "loved her with all (his) heart" and some sick dynamic Hailey was trapped in between the two of them, he may want her close by for his reasons. Billie may want her close by. Billie has couched Hailey in terms of belonging to her enough times that she does have some aspect of maternal feeling even if the most base.