Why is water hard to heat?

Because the evaporation of water carries heat away from the fire.

Water has a high specific heat capacity. This is a measurement of the amount of energy required to cause a unit volume of a material to increase by a unit temperature. As such you must add a lot of energy to water to cause a small increase in temperature.

For example to increase the temperature of 1 litre of water by 1 degrees C, you must add 4181.3 joules of energy.