The airline is Boeing's biggest customer and had 34 of the planes in its fleet - 4.5% of the total - when they were grounded on March 13.
Aside from that, the Southwest and American, had also been taking heavy hits in to their profits as they had to grapple with a growing number of passengers during holiday seasons with slimmer fleets.
The domestic-focused U.S. carrier, which had previously set February 8 as a return date for the MAX, said "proactively" removing the aircraft from service reduced the likelihood of last-minute cancellations and disruption.
An internal review by the Federal Aviation Administration (FAA) also found regulators often deferred to Boeing on questions of safety, allowing the company to conduct safety testing largely on its own. Southwest had originally grounded the plane through February 8, and has now extended that grounding through March 6, the biggest delay from a US carrier.
The airline cited "continued uncertainty around the timing of MAX return to service" in its decision to extend cancellations by another month, the longest delay for any USA carrier.
American Airlines then announced it expected to the MAX begin flying again on March 5, 2020. Boeing has confirmed it must submit revised documentation. The FAA said it would take 30 days after the certification flight before it would unground the plane. according to Reuters.
It is the largest customer of the aircraft in the United States with 34 in its fleet and 200 orders pending when air authorities made the decision not to allow the planes to fly in the wake of the two fatal accidents.
The company has had discussions with Boeing regarding compensation for damages due to the Max groundings, but said on Friday that no conclusions have been reached.
Southwest Chief Executive Gary Kelly has said he is "not happy" with the MAX situation.
As a result of the extended cancellations, Southwest said it was unable to provide an update on first-quarter capacity guidance at this time.