why are diff. years of american eagles priced differently?

Some vendors have better prices or lower mark-ups than others, and some just charge for silver, and others charge for collectiblity.

The normal answer would probably be that the out-of-production earlier year-dates are collectible to some extent, more than the current production year-dates. Just like in alot of coin collecting circumstances.
The years with lowest production are rarer, and command higher prices on the collector market.
However, if a circumstance is seen where this year's coin production is slated to be a very low production year, that could cause current production coins to actually be costlier than older common-date coins.

For a person just interested in the silver content value, don't buy the collector coins which have the premium mark-up for their collector value.

I guess some may be more collectible than others.

That's what the free market wants to pay for them. Oh wait. What free market? Nevermind, I don't know.

I would buy the cheapest years. It will be their loss. In light of all the manipulations that are going on. I would not buy precious metals with the hopes of making a profit in the current system; however, I would buy precious metels to use after the crash. Hopefully then more people will start to understand sound money. Just make sure you have your pantry stuffed with food before any other investment. Not everyone will understand sound money even after the crash, but everyone will understand the pains in their stomach.