I disagree for the simple fact that earnings could be increased through tactful negotiations with the PUC. "It costs us x more to put everything underground, plus, it will increase our maintenance costs by y per month". Meanwhile, that 'y' would not be seen as an actual accrued cost for MANY years during which earnings would appear to blossom. Then, when it DID hit, they would go back to the PUC to ask for another increase due to the increased costs of maintenance. By then, the greater earnings would be seen as normal for that company and they could make the case at that point that they shouldn't be punished for increased costs incurred. Like it or not, this is frequently how companies like this increase their earnings on their "regulated" industry status. The only other way is to absorb other companies, and that is a costly (and typically unprofitable) venture.
True costs, however, that are not allowed to be passed on to the customer, though, do cut into earnings, and underground is ALWAYS far more expensive for the electrical industry. It is also far more dangerous, believe it or not due to confined spaces, poison gas, oxygen deficiencies, and many other hazards completely unrelated to electricity. In fact, it is not uncommon for an electrician to don something akin to a scuba suit just to do their job underground. These costs can not be passed on readily because they are considered one time equipment costs. This cuts into earnings while not being accounted for in the payments.
So I guess indirectly we are saying the same thing, just looking at two different sides, but I still stand by costs as being the driving factor as that can not be regulated while earnings are limited by the maximum charges the PUC allows.