The reason why mountain bikes are harder to pedal is mainly because of the low tire pressures and geometry.
Mountain bikes also have thicker tires, different gearing and a heavier frame than other bikes which also make the mountain bike harder to pedal.
As a result of the thicker tires, heavier frame and different gearing of the mountain bike it makes the mountain bike slower and requires more energy to pedal than a regular road bike.
Mountain bikes also typically have wider, knobbier tires that are designed for traction on uneven terrain.
These types of tires create more rolling resistance on smooth pavement, which make it harder to maintain speed compared to the thinner, smoother tires of road bikes.
Mountain bikes can also be used for both, road and trail riding, which is why mountain bikes are a great option for someone who is into off roading as well and urban riding.
When comparing their prices, mountain bikes are also in general the expensive option out of the two and this is because they have more specs and features.
On a mountain bike a small chainring, such as a 28t or 30t, will give you the lowest and easiest possible gear on your mountain bike for climbing.
A big chainring, such as a 34t or 36t, will enable you to pedal your mountain bike faster in the highest and hardest gear.
Most mountain bikes also come with a 32t chainring as standard, because it provides a solid middle ground.