Why can't he just sign now?
That's not true, since the Stars own his rights, he can sign whenever. Its weird that they're waiting until July 1st to announce it, because they can sign him right now with no issues.
Actually he can't sign now, has to wait until July 1st.
There are only three situations where teams are allowed to sign players to contracts that don't go into effect until the next season, which starts on July 1st.
a) contract extensions
b) unsigned draft picks - can be signed between March 1st and June 1st
c) draft-related unrestricted free agents - can be signed between March 1st and June 1st
Nichushkin does not fit into any of these three categories. Yes, he is an RFA, however he is not under contract at this time. Meaning he is not allowed to sign a new contract under the (A) contract extension exemption.
And to expand on that, Nichuskin cannot be signed now for the 2017-18 season as there are no games left he would be eligible to play in.