They don't all burn out at the same time, and a few have lasted quite a while. I think they were wired in parallel, since some go out and others further down the wire stay lit. The wattage is 50 wattsSounds like too much load possibly, since the bulbs are acting like a fuse. What wattage are the 8 bulbs. Also, are they all burning out at the same time, or different times. Also, how did you wire the circuit, because other thing that comes to mind is that you may have wired it incorrectly.
AnswerWhat voltage are they getting? If you remove one lamp, turn on the switch, then measure voltage at the empty socket then you will get the voltage for all of them (within plus or minus one volt which is inconsequential).
A loose neutral will cause overvoltage from time to time and this will greatly shorten the life of lamps of any kind. Undervoltage will not shorten the life of incandescent (including halogen) lamps and in fact slows down the progression towards burnout. Accidentally wiring them in series results in undervoltage.
Is there vibration from the floor above where the lights are installed? This can shorten the life of the lamps.