Nine players from last year missed out, with six new players included.
Foster said most of the selectors' time was spent choosing their halfbacks and props.
Aaron Smith, Finlay Christie and Folau Fakatava were the halfback choices.
Props Karl Tu'inukuafe, George Bower, Nepo Laulala, Ofa Tuungafasi and Angus Ta'avao were joined by new cap Aidan Ross.
Foster said: "In our opinion, we had five nines [halfbacks] playing really, really well, offered something different, and this is a good chance to see what Folau and Finlay can bring.
"Folau offers something around the edges of the ruck that's pretty unique, yet we don't lose anything in the defensive area. He's strong defensively. He likes to go and fight, probably a little too much, at the breakdown, but that's a good thing to fix.
"They are the key reasons that he's been picked," he said.
"There's another team being named soon [Māori All Blacks] so maybe there's another chance for a couple of others to put their hand up," he said.
Smith and fullback Jordie Barrett both had groin issues, but it wasn't known to what extent before the squad assembled.
There was a group of young props coming through who the selectors were very excited about, but Ireland's strength in the set-piece and its all-round ability, meant they were delighted to have Bower and Ross on the loosehead. Ross had gone up a couple of cogs at scrum times.
He was a consistent scrummager, and both he and Bower were relatively new at Test level, so that had contributed to Tu'inukuafe's inclusion.
"He is our strongest loosehead scrummager and we think for right now he is the best way to support the other two players," he said.
Highlanders prop Ethan de Groot missed out, and Foster said they believed he could get fitter and also needed to be quicker about getting off the ground and involved.
"We've got a high regard for him but we'll put a plan in place for him to give him an opportunity to come back," he said.
Foster said he spoke with all nine players from last year who hadn't made the squad and, while disappointed, he could hear their passion on the end of the phone.
Each of them would be keen to get back in, and the selectors were conscious of the need to give them the plan they needed to achieve that.
Foster said league convert Roger Tuivasa-Sheck was getting better.
"If you look from the start of the campaign to now, he's improving. He's improving in a whole lot of aspects. I thought a few weeks ago, particularly against the Brumbies in the round-robin, we saw someone put his hand up and carry well in that midfield position.
"He's a good defensive reader and I think, by his own admission, he's got areas he needs to get a bit better at – his distribution game when he's in the middle of the park, his kicking game we haven't seen a lot of yet," he said.
There were a few little things he needed to get right, but they felt the time was right to bring him in and start working with him.
Wing Leicester Fainga'anuku had been on the selectors' radar for a while, and they saw him as a wing who could probably cover centre but was more of a wing.
"He just brings power to that position, he backs himself with the ball," he said.
Some questions would be asked of him at Test level with the high ball, but he had the energy and ability to break things open and gave them some selection options.
First five-eighths and fullback Stephen Perofeta had his form this season to thank for his selection.
"We've liked Stephen for two or three years. He's had some niggles. He's had a few wobbles with his goal-kicking and a whole lot of things, but hasn't he shifted his game this year?
"The work that Leon [MacDonald] and probably Beaudy [Beauden Barrett] have done alongside him has been outstanding. He's settled, he's confident, he's playing at well at 15 [fullback] as he is at 10 [first five-eighths] and goal-kicking well," he said.
Foster said he also could create space close to a line with his feet. He had great composure under line-speed pressure. He also gave them another option when selecting a 23 for a Test.