What if the 7 Express did stop at 74th St?
On the 7, express and local trains run at about the same frequency. Since the waiting time is going to be the same for either a local or an express train, you’d just want to take the fastest train out of your station heading to your destination. The only time a local train would be faster than an express train would be if you were heading to or from a local stop.
In 2016, the express stations (Queensboro Plaza, Woodside, Junction Blvd, Mets-Willets-Point and Flushing-Main St) saw 112,482 passengers every weekday. Local-only stations east of Queensboro Plaza — excluding 74th St — saw 89,959 passengers every weekday. The four express stations stations are much busier than the rest of the ones on the line, and on top of that Queensboro Plaza’s ridership statistics do not count transfers to and from the N/W. (There are quite a lot of them; the “peak load point” where trains are most crowded on the 7 is Queensboro Plaza because the 7 is much slower going into Manhattan than the N/W.)
Because express trains would serve the busiest stations much faster than local trains, and because they have roughly similar wait times, expresses would enter and leave 74th St severely overcrowded, and locals would be relatively empty. This would be unpleasant for everybody; for commuters, express trains would be delayed as lots of people tried to get on and off packed trains. For the MTA, operations would be inefficient; some trains would be packed and have to leave passengers behind, while others would leave the station with plenty of room to spare.
You can actually see this in effect downstairs at Jackson Heights-Roosevelt Av; the platforms during peak hours are overflowing with people being passed up by express E/F trains, while local M/R trains pass through with seats available.