wayland, well it's certainly possible you're not getting deep enough, or perhaps it's a soil type that's simply causing the old stuff to be out of reach? For example: If you've found clad dimes and '70s pennies at 8 or 9", ......... well then that's a bad omen that the soil type (very moist perhaps) is putting the older stuff out of reach.
But if you've not found clad over 3 or 4", well then perhaps that's not the issue.
I would be inclined to agree with inspector gadget: you simply don't know who's pounded that park in the past. Perhaps you think detecting is not big there now, where you are, but you simply don't know if this was true 10 yrs. ago, 20 yrs., or 30 yrs. ago. And simply because you (or persons you talk to who lived there in decades past) say "I've never seen anyone md'ing this park" doesn't mean a hill-of-beans! Because the average person who's not into this hobby, simply doesn't notice it. It's only us geeks who notice someone else md'ing

I've even heard persons who live right across from schools or parks that I KNOW FOR A FACT have been pounded to smithereens, say: "I've never seen anyone detecting there", blah blah. Well they simply don't watch it at all times, and simply wouldn't make note of something like that (for permanent recall), since it's not a hobby they're into.
Back in the late 1970s and early 1980s, when silver hit all time highs back then, and ground-breaking machines like the first 6000d made silver easy (added an instant inch or two, and effortlessly cut through minerals), there was a sort of "silver rush" mentality. Whereas NOW I would *bristle* at hunting a park that's *merely* 1940s origin. Yet back in those days, we made sport of hitting any park or school that was 1960 or earlier, and cherry picking the silver and wheaties. I can think of parks and schools that are '40s and '50s origin, that we used to get silver out of, that you could NEVER get silver out of now, even if you're an ace. We simply covered them thoroughly and depth wasn't the issue back then. So pity-the-poor soul who goes to some of our old stomping grounds today
But it's a fairly regional thing though, because in other areas, with less md'ing, perhaps good parks (that give up oldies to proficient users of the current breed of deepseeking machines) can still pull oldies up. But for sure, other parks are just sad cases, filled with nothing but current clad nowadays.