Because that’s not how it works. For a variety of reasons. Mainly because you don’t typically chase a second accolade while you currently have one. Double champions are rare and few and when it happens, they lose one or both quickly. It’s not a long term situation.
The bigger reason is you can’t sell someone as being ready for the next step while they’re still bogged down by low card angles and accolades.
WWE wouldn’t push the 24/7 Champion into the main event, for example. He’d have to lose it first and do some serious damage work to elevate him beyond that division to the fans.