If you were passed over in the draft, why could you not be signed now?
Just the rules of the CBA. There's a window to sign an undrafted (North American) player before the NHL season starts, but once the window is closed they are back in the draft pool. Here's the CBA section in case you were interested (section 8.9 (b)(ii)):
8.9 Eligibility for Play in the League. No Player shall be eligible for play in the League unless he: (a) had been claimed in the last Entry Draft, or was ineligible for claim under Section 8.4; or (b) had been eligible for claim in the last Entry Draft, but was unclaimed, and:
(i) had played hockey in North America the prior season and was age 20 or older at the time of the last Entry Draft, and signed an SPC which was signed and registered with the League between the conclusion of the Entry Draft and commencement of the next NHL Season.
(ii) had played hockey in North America the prior season and was under age 20 at the time of the last Entry Draft, and signed an SPC which was signed and registered with the League between the conclusion of the Entry Draft and commencement of the Major Juniors season (except that if such Player had signed an NHL try-out form, which was signed and registered with the League during the aforesaid time period, then the deadline for signing and registering with the League an SPC with such try-out Club shall be the commencement of the NHL Season).
(iii) had played hockey outside of North America in the prior season and was age 22 or older at the time of the last Entry Draft and signed an SPC which was signed and registered with the League between the conclusion of the Entry Draft and the commencement of the next NHL Season.
(iv) The words "eligible for claim in the last Entry Draft" in subparagraph (b) above mean "eligible for claim in all rounds of the last Entry Draft." The words "the prior season" in subparagraph (i), (ii) and (iii) above mean "a full season prior to the last Entry Draft."
The CBA language always gives me a headache but also gives me a little sympathy for when the Oilers tried to sign Vladimir Tkachev. By the definition of 8.9(b)(ii), it seemed like he was eligible to be signed but a different section of the CBA (8.4) said he wasn't since it said he had to have played the entire year in North America not just part. Here's that chunk:
8.4 Eligibility for Claim. (a) All Players age 18 or older are eligible for claim in the Entry Draft, except:
(i) a Player on the Reserve List of a Club, other than as a try-out;
(ii) a Player who has been claimed in two prior Entry Drafts;
(iii) a Player who previously played in the League and became a Free Agent pursuant to this Agreement;
(iv) a Player age 21 or older who: (A) has not been selected in a previous Entry Draft and (B) played hockey for at least one season in North America when he was age 18, 19, or 20 and shall be eligible to enter the League as an Unrestricted Free Agent pursuant to Article 10.1(d); and
(v) a Player age 22 or older who has not been selected in a previous Entry Draft and shall be eligible to enter the League as an Unrestricted Free Agent pursuant to Article 10.1(d).
So if you were ever wondering why there'll be a handful of 21 year old Europeans drafted every year but 21 year old North American can sign as free agents, it's because the CBA requires an extra year of eligibility for European (trained) players.
An edge case off the top of my head: Czech goalie Josef Korenar went undrafted in his original draft year in 2016. Then he played in the USHL in 2016-17. San Jose was going to draft him in the 7th round but ended up trading that pick to move up in the 6th round. Korenar went undrafted again, but since he played a full season in North America, San Jose could sign him to an ELC that summer even though Korenar could have been 2018 Draft eligible.
Czech defender Radko Gudas went undrafted in 2008 and 2009. He attended Kings prospect camp in the summer of 2009 and impressed. LA would have loved to have signed him to an ELC, but they weren't permitted since he hadn't played in North America yet (Gudas was going to play in the WHL in 2009-10). So Gudas went back to the draft pool and was a 3rd rounder to Tampa in 2010.
And then for a North American comparison, Torey Krug (1991 birthday) went undrafted in 2009-11 and was no longer draft eligible for 2012. So he was free to sign with Boston. Tanner Pearson (1992 birthday) was still eligible one last time for 2012 and was a late first rounder by Los Angeles.