The defeat at Celtic Park saw the Scottish champions dumped out of the competition for a second time this season, having only been reinstated following Legia Warsaw's punishment for fielding an ineligible player.
Maribor's 2-1 aggregate triumph was sealed by a Marcos Tavares strike 15 minutes from time on Tuesday, leaving Celtic with plenty to ponder.
And Deila, whose side also lost to Inverness Caledonian Thistle in the Scottish Premiership on Saturday, offered no excuses for failing to reach the group stages.
He told the club's official website: "They got two chances during the game and they were more effective than us, but in the end there is only one thing to say and that is we weren't good enough and we didn't deserve to go to the Champions League.
"The pressure was too little and there was not enough aggression, but in the second half we were more aggressive and we got some chances as well and it was an even game.
"We were ready to go and we wanted to go hard and pressure them but I think, again, we were a little bit frightened and wanted to protect too much."
Celtic will now go into the draw for the Europa League, which will take place on Friday.
'Obviously Messi is the best player in history and being compared to him means that you're doing things right, but I try to be myself': Barcelona star responds to comparisons with Argentine legend
'Darwin Nunez would be gold for Arsenal': Liverpool legend claims Gunners should have signed Uruguayan striker in January
