Why does the US claim it's a democracy if Mexicans can't vote in the US, even if they live there? It's because they're not citizens. Similarly, Palestinians living in the West Bank are not citizens of Israel. Other people, including many Arabs, that are citizens of Israel, do vote.
I'm not saying you're necessarily wrong btw - there's a lot of valid questions around whether it's OK or not that Palestinians are not citizens of Israel even though Israel controls their land (e.g. right of return, the idea that Israel should've annexed the land, etc). I'm just pointing out that what you bring up as a rebuke of Israel really isn't.