Even these days the demand for electricity "on tap" is still low so there's no great need for airports to provide electricity
Perhaps not for the airports, but it's something that the airline should consider. Certainly Southwest thinks there's something to the idea or they wouldn't have spent the money to do what they did, right? Is electricity alone going to be a huge factor for people's airline decision making? No, but in combination with other customer-friendly perks, it builds loyalty and affinity.