When an oilfield was nearing the end of its profitable life, it was common practice for a major oil company to sell that field to a upstart junior oil company. The ostensible financial reason was the junior oil company has less overhead, thus it could afford to keep all those pumps working and still make a profit where the major could not. To some degree, this worked: the junior company could run the field a few years longer than the major company.
When the profits were extracted, the junior oil company just declared bankruptcy, with the owners having extracted all the profits beforehand. With no owner for these wells, there is no one to the cleanup.
The change of ownership only allowed the major oil company to rid itself of its liabilities. The junior company had no intention of cleaning up.
This practice has been going on for at least 40 years. The Alberta government is just waking up!