There are as many reasons why the kit was discontinued as there are discontinued kits. Many are worth about what you paid for them, some might be worth more, most will not be worth what you paid back then.
I doubt there are any "valuable" old Italeri kits with the exception of a handful of old Peerless Max reboxes they did like the old Dodge WC ambulance. It is more hard to find and sometimes gets a nice price from someone who really wants it.
Many discontinued Tamiya kits were due to the original molds being retooled to update the model. For instance, the M1 Abrams was retooled into an M1A1 so the older 105 mm gunned kit has become hard to find. Same with the M2/3 Bradley. It was retooled into the M2A2 and retooled again into the M2A2ODS so the older version is more rare, but also more outdated.
Their WW2 German tanks were just outclassed by newer kits by themselves and other companies so the value wasn't there. Why spend $30-40 on an old Tamiya kit when a more accurate, better detailed kit by another company can be bought for less?
For researching which kits have value, eBay can be your friend. Do a search of their site for completed auctions (ones that someone bought the kit) to see how much people paid including shipping. Don't just check what they list for. You may have a few kits that are worth a lot and 95 that aren't worth anything. And packaging is very important too. If it is valuable in one box, it might not have the same value in another.
Several years ago, I listed a bunch of the motorized off scale kits by Japanese companies like Tamiya, Nitto, etc. on eBay. I made nearly a thousand dollars for these kits I paid between $5-10 for. Buyers from Japan and the US were the heavy bidders.