Wikipedia suggests this was because he was already sentenced to double life imprisonment. Clearly prosecutors should not waste time pursuing charges that won't really impact a criminal's status, do you disagree?
In one indictment, in a New York federal court, he was charged with several crimes, but not murder-for-hire. In the other indictment, in a Maryland federal court, he was charged just with murder-for-hire.
The case in New York went to trial first. The murder-for-hire evidence was introduced in that court as part of trying to prove some of the elements of the other charges.
For example to prove a charge of engaging in a continuing criminal enterprise one of the elements is that the person occupied "a position of organizer, a supervisory position, or any other position of management". Evidence that a person is trying to hire hitmen to protect the enterprise is evidence that they occupy such a role.
After the convictions and sentencing in the New York trial were upheld on appeal, the prosecutors in Maryland dropped their case because he was then in jail, for life, with no possibility of parole. They said it was a better use of resources to focus on cases where justice had not yet been served.