The White House is adamant the decision is based off security concerns and is claiming the previous policy poses a risk to overall military effectiveness.
“The president’s expressed concerns since this Obama policy came into effect, but he’s also voiced that this is a very expensive and disruptive policy and based on consultation that he’s had with his national security team, came to the conclusion that it erodes military readiness and unit cohesion and made the decision based on that,” stated Press Secretary Sarah Sanders.
https://www.oann.com/justice-dept-asks-supreme-court-to-allow-transgender-military-ban/