Scola makes $4,508,504 this year and $4,868,499 next year. His contract next year is not guaranteed, which is why waiving him clears up space for Lance.
Bird gave up all that for Scola because he wanted to go for a championship this year and he wasn't getting that done with Green or Plumlee. If he wants to continue to contend, he's not going to do that by keeping 34-year-old Scola over 23-year-old near-All-Star Lance.
Look, I like Scola. But Lance is not only more important NOW but also in the future.