So first, it seems a little strange that after an entire year of work, they would suddenly be "too busy to take on undergrad students." Of course I am only speculating, but my guess is that they (rightly or wrongly) didn't think your project was going anywhere and decided to cut you loose. This would also explain why they didn't ask for your data or notes, and why they have since hired new undergraduates. If this is the case, this PI would probably not be a good choice when asking for letters of recommendation.
At any rate, to your questions:
I'm not sure if maybe my project as a undergrad wasn't that important, or if I was unprofessional.
If they wanted your data or notes, they should have asked for them. I assume there was no directive that you failed to follow, such as "upload your results to the portal every week." So no, I don't think it was unprofessional.
Why didn't they ask for your data or notes? Two possible reasons:
- In my experience, first-time undergraduate researchers do things literally hundreds of times slower than I can. So, I would likely start from scratch rather than trying to turn the student's product into something useable. Of course, this will vary by field -- I can imagine that some "wet labs" have very time-consuming processes for which undergraduate data would be valuable (if we trust the undergraduate).
- Everyone always has more ideas than they have time. If an idea gets consigned to an undergraduate, it's probably not very high-priority. So, it could be that no one is pursuing that idea, or that a new undergraduate will start from scratch on it.
Would it be late now, more than a year after I left, to send in data?
Personally, I would not offer. As noted above, this is unlikely to be useful; further, it raises the question of "if you thought I needed this, why didn't you provide it a year ago?"
I was hoping to ask the PI for letters of rec for grad school but I haven't spoken with him for a while and I'm not sure if I left a bad impression when leaving the lab
I wouldn't worry about not having spoken with him in a while -- everyone is busy. And I wouldn't worry about the data/notes situation. But I would think carefully about the circumstances under which you left the lab -- generally, we don't fire students if we think they deserve a very strong recommendation. But of course there are exceptions. You can certainly ask "Would you be able to write a strong recommendation?"