Flight "loads" are largely meaningless at this point as most of these flights have very few seats booked this far out. What matters is fare bucket inventory availability which is controlled by revenue management systems and are based on historical demand patterns, major events/holidays, etc. DL uses P/A/G fare classes for PS with the cheapest being G. There are summer seasonal G fares for $481.50 each way on ZRH-ATL and $394 each-way on ZRH-JFK which have a 7-day minimum stay requirement. In order to book them, you need to find flights with G bucket availability. Note that it's "Schedule Change Saturday" today as DL tends to load schedule updates on Saturday mornings. This can cause issues with inventory management and you are probably better off checking again tomorrow.

As noted above, DL91 only has A bucket open on July 22nd and P on July 23rd (although the July 22nd flight is only showing P fares for booking for some reason). There is G bucket open on DL91 on July 16th. DL52 JFK-ZRH returns on Aug 12th and 13th both have G bucket availability.