It has nothing to do with boost.(Kind of)
The real reason is half assed tuning. Most Turbo cars can be quite reliable. However tuning is quite costly and of course this is where most people skimp out.
Also turbo/supercharged motors are usually modified to make much more power than stock. sometimes over 200HP/L. If you compare a high strung n/a motor to a turbo/supercharged motor you likely wont see a big difference in reliablilty. Comparing a stock turbo car to a stock n/a car I can imagine the reliablility being very similar.
edit: Damn you Forbin!