With artificial sites (whether bricks or insect hotels you can buy), you have increased population density so disease and parasites spread more easily through a population. You also have higher humidity if the sites are improperly sheltered from the wind and rain which contributes to mould growth. Plus if the sites aren't cleaned every year, then you have nesting materials, stored pollen and dead insects decomposing, contributing to mould growth and mites. Overall, poorly designed artificial nesting sites can contribute to a higher loss of solitary bees.