First they feed almost exclusively on humans, so if they pick up a disease from one person, they are likely to pass it on with their next bite.
Other mosquitoes are less efficient spreaders as they feed off a variety of animals.
These particular blood suckers also drink little and often.
Rather than gorge their entire meal from one person and running the risk of being swatted they graze from multiple people making it easier to spread disease.
They are also active during the day so bed nets are less effective.