MAVS NEWS:

[F Gump]

That all makes complete sense, but I was asking why they didn’t go ahead and sign him instead of Kelley in the first place? He’s a guy they know. Has some familiarity with system. Has produced at least on a small level in the NBA. Perhaps they thought they could catch lightning in a bottle with Kelley as opposed to the known commodity that is Brown. Who knows?

It's apples and oranges.

Kelley is a two-way player (they can have up to 3 of them), and costs $0 off of their cap. 
Brown is a 15-man roster signing whose salary is paid out of the cap payroll, which is limited by the hard cap.  Brown is not eligible to be signed to a two-way contract.

After the deadline trades, the Mavs have only had enough cap money to pay a single 15-man roster player for 15 days. The smallest possible contract is for either 10-days or "rest of season."

Brown is not replacing Kelley afaik. If they wish, Kelley can stay on the roster to the end of the season at no cost to the cap payroll, and I assume he will continue to be here until and unless the various centers get healthy and he serves no purpose. He can be replaced by a different two-way player as late as March 3.