I think he cared about Brooke but that's not who he wanted at the time, it was Peyton. It's not about wanting Nikki over Brooke but knowing that he could/was hurting Brooke while whatever he had with Nikki was casual. Lucas has always need a girl to fill the void when he was alone. He could not have the girl he truly wanted, so he didn't want a commitment/girlfriend and Brooke was way more than just a fling at that point.
And honestly I think it showed that Lucas at 17 was way more mature than Lucas at 22. Because his S1 relationship with Brooke was very similar to his S5 relationship with Lindsey. He couldn't have Peyton so he pulled another girl into his mess, played her and used her just because he could not have Peyton.