Evans gambit is not popular anymore.
The problem with the Evans gambit comes when Black accepts the gambit since he can return the gambit pawn in proper moment to defuse White's attack.
This is the crucial line:
1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.b4 Bxb4 5.c3 Ba5 6.d4 d6 7.Qb3 Qd7 8.dxe5 Bb6!=
Until this day nobody was able to find edge for White-that is why nobody plays it anymore.
I looked at it and I don't see anything wrong with it. I can't go over all the variations here, but you could look for an article or a Youtube video
YouTube and find opinions of the strongest Grandmasters.
Quality of those videos is usually low, as they intend to get subscribers by showing them attractive variations instead objectively the best lines.
Not a single theoretical main line was examined properly-only sidelines that give Black poor play.
I can only see black losing, so what's wrong with Evans Gambit declined?
Declining the gambit is not the best option for Black. White has no problems since he gets the initiative because his opening idea was fulfilled. He will use his free tempo to further hinder coordination of the Black pieces.
Still, Black has good play in all lines, but must defend first for a long time, and attack later.
Am I missing a variation?
If all your sources provide as "quality" coverage as the video you linked to-then yes, you do miss a lot.
I will quote the Chess Informant ECO 2006 for the lines in Evans gambit declined:
LEGEND OF SYMBOLS:
+/= Minimal advantage for White
=/+ Minimal advantage for Black
+/- White is better
-/+ Black is better
=/∞ Compensation for material
+- White wins
-+ Black wins
1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.b4 Bb6 (4...d5 5.exd5 Nb4 6.O-O Nf6 7.Nxe5 Nbxd5 8.d4 Bd6 9.Bg5 c6 10.Nd2 O-O 11.Qf3 h6 12.Bh4 Be6 13.Rab1+/=) 5.a4 ( 5.Bb2 d6 6.a4 a6 7.b5 axb5 8.axb5 Rxa1 9.Bxa1 Nd4 10.Nxd4 exd4 11.c3 Nf6 12.O-O O-O 13.d3 d5= ) ( 5.b5 Na5 6.Nxe5 Nh6 ( 6...Bd4 7.Nxf7 Qf6 8.Qe2 Nxc4 9.Nxh8 Ne5 10.Nc3 Ne7 11.Bb2+/= ) 7.d4 d6 8.Bxh6 dxe5! 9.Bxg7 Rg8 10.Bxf7+ Kxf7 11.Bxe5 Qg5 12.Nd2 ( 12.Nc3? Nc4-/+ ) 12...Be6 13.Qf3 Ke7 14.O-O-O! Raf8 15.Qa3+ Kd8 16.f4 Qe7!? 17.Qxe7+ Kxe7 18.g3 Bxa2∞ ) 5...a6 ( 5...a5 6.b5 Nd4 7.Nxd4 ( 7.Nxe5 Qg5 8.O-O ( 8.Bxf7+ Kf8 9.Ng4=/∞ ) 8...Qxe5 9.c3 Ne6 10.Re1 Nf6 ( 10...Bxf2 11.Kxf2 Qc5+ 12.d4 Qxc4 13.Re3 Nf8 14.d5 Qc5 15.Ba3 Qb6 16.d6=/∞↑) 11.d4 Nxd4 12.cxd4 Qxd4 (12...Bxd4 13.Ra2=/∞ ) 13.Qe2!? (13.Qxd4!? Bxd4 14.Ra2=/∞ ) 13...Qxa1 14.e5=/∞↑ ) 7...Bxd4 8.c3 Bb6 9.d4 Qe7 (9...exd4 10.O-O! Ne7 11.Bg5 h6 12.Bxe7 Qxe7 13.cxd4 Qb4 14.Na3 O-O 15.Qd3 d5! 16.exd5 Bd7 17.Nc2 Qd6 18.Ne3+/= ) 10.O-O ( 10.Qg4 Nf6 11.Qxg7 Rg8 12.Qh6 Rg6∞ ) 10...d6 11.f4 ( 11.Be3!? ) 11...Be6 12.Na3 exd4 13.cxd4 O-O-O 14.Be2 Nf6 15.Bf3 d5 16.e5 Ne4 17.Nc2 h5!= ) 6.Nc3 Nf6 7.Nd5 ( 7.d4 Bxd4!? ( 7...exd4 8.Nd5 Nxd5 9.exd5 Qe7 10.Kf1! Ne5 11.d6 cxd6 12.Bd5=/∞ ) 8.Nxd4 Nxd4 9.f4 ( 9.O-O d6 10.f4 Be6! 11.Bxe6 fxe6 12.Be3 Nc6 13.b5 axb5 14.axb5 Rxa1 15.Qxa1 Ng4!? 16.bxc6 Nxe3 17.Qa2 Nxf1 18.Qxe6 Qe7 19.Qc8+ Qd8 20.cxb7 O-O 21.Nd5! Kf7 22.Qf5+ Kg8 23.Qc8= ) 9...d6 10.O-O O-O 11.fxe5 dxe5 12.Nd5 Be6 13.Nxf6+ gxf6 14.Bd3 Qe7 ( 14...Nb3? 15.cxb3 Qd4+ 16.Kh1 Qxa1 17.Qf3!+-->) 15.Bh6 Rfd8∞) 7...Nxd5 ( 7...Nxe4 8.O-O d6 9.d3 Nf6 10.Bg5+/= ) 8.exd5 Nd4 ( 8...e4?+/- ) 9.a5 Ba7 10.d6 ( 10.O-O?!=/+ ) 10...cxd6 11.O-O O-O 12.Nxd4 Bxd4 13.c3 Ba7 14.Qf3=/∞
How could black win against Evans Gambit declined?
How could he not win?
In all the variations above Black has sufficient counter-play and equal chances to win.
He will first try to parry White's initiative, and then he will have won endgame due to his healthier pawn structure and White being overextended-this is a standard plan for Black in most openings.
Evans gambit is not popular anymore because White does not get the attack he hoped for, and after Black properly returns the pawn he has better or at least equal chances in the endgame. Why would anyone play with White just to fight an uphill battle to draw?
As for the declined variation, White will get lasting initiative but Black still holds. In all lines quoted above Black has sufficient counter-play and both players need to play accurate in order not to lose.
For a detailed coverage of the Evans gambit find repertoire books like The Great Evans Gambit Debate series or Play the Evans gambit-Tim Harding.
Hopefully this answer sheds some light. If you have further questions leave a comment and I will try to respond as soon as possible.