Those Mercury capsules pulled a lot more Gs on reentry than the Shuttle
If memory serves, the Mercury astronauts pulled about 9G on reentry. The Shuttle pulls somewhere around 3G.
Higher G loading leads to higher structural weight, meaning lower payloads to orbit or a much bigger booster. For every additional kilogram you want to place in orbit, you need several additional kilograms of fuel and structural weight at liftoff. As a result, the takeoff weight tends to snowball quickly if weight control isn't maintained. There is no free lunch in all of this. Everything in aerospace vehicle design is a tradeoff.