Why is selection less effective in small populations than in larger?
This question seems to be about negative/purifying selection, judging by the wikipedia quote. Positive selection can often (but not always) occur more slowly in smaller populations, but 'efficiency/effectiveness' of selection generally means the rate at which deleterious variants are purged from the population, so I'll skip out taking about positive selection (unless you did mean that).
First, genetic drift: Genetic drift is the stochastic change in allele frequencies caused by random differences in the survival of different individuals. For example, a mouse might get struck by lightning and not transmit any of its genes to the next generation. Whether or not it was struck by lightning probably wasn't related to its fitness, it was just 'bad luck'. As population size increases, genetic drift becomes weaker because the larger the population, the smaller the proportional impact of each random event that concerns just one individual
Negative selection tends to be less effective in smaller populations because random fluctuations in allele frequency (drift) may lead to deleterious alleles reaching higher frequency. The rate at which deleterious alleles are removed from a population depends on mutation, dominance, linkage, and demography and can vary across populations. Therefore, the average fitness over time may become reduced. Kimura in 1963 was one of the first people to show this.
In reality, 'low efficiency of selection' and 'strong drift' in small populations are the same thing.