Guys, drivers don't own their car numbers in NASCAR... unless they're also the team owner.
NASCAR sells numbers to teams. Teams can then sell a number and obtain another one, with NASCAR's consent (which is usually a rubber stamp).
Just because a racer's relative made a number famous doesn't mean it gets passed on to him. If I was in the position to drive in NASCAR and had a famous relative who had driven in NASCAR, I'd rather use a different number so I could establish my own identity and somewhat lower the stress of fans' expectations.
For the number 9, I don't remember who used it last before Elliot (it may have been dormant for a while) but apparently Elliot requested it and got it. But during his career, to keep the lights on, Elliot sold his team to Melling and the #9 rights went to Melling. That's why Elliot had to come up with a new number (#94) when he established a new team (after disbanding his old team and driving for Junior Johnson for a while). Melling continued to field a #9 car with Lake Speed being the most notable driver. I think Elliot used #94 because his nephew Casey had used it (and as a tribute to Casey as he was battling cancer).
Melling eventually quit racing, and later sold #9 to Ray Evernham's start-up team. When Evernham merged with and then sold out to Petty the #9 came along with the deal. So Petty owns the number 9 (in Sprint Cup) and can resell it if he wants. That number still has some value since Petty has continued to use it and to attach driver names and sponsorships to it.
If you really feel that Chase Elliot should run #9 in Sprint Cup, write to Petty Enterprises. You never know, Petty may be willing to give it up or sell it. But with NASCAR controlling numbers issued and teams having a monetary stake in the numbers they use, don't expect that any driver "deserves" to get the number used by their famous relative. If Jr. couldn't pull that off, drivers with a smaller fan base are even less likely to.