If a machine is capable of multitasking, why is only one program running while
the others are sitting there ?

I can't work out how this relates to the current thread.

It's quite difficult to find a machine that is not capable of 
multi-tasking.  Did you really mean one which has multiple processors,
or did you mean running on an OS that supports multi-tasking on the
processor?

In general one would not want to multi-task typical distributed
computing applications, on a single processor, because multi-tasking
works by continually switching between the two and there is a cost in
performing the switch, and in the increased cache memory demands.  If the
applications are such that they often require rather different resources
(typically I/O versus CPU) multi-tasking can allow both resources to
be more fully utilised, but typical distributed computing applications
dominantly use CPU time as their only resource and do not benefit.
SETI@Home is also quite a heavy memory accessor, so sharing the L2 cache
will also compromise its performance.

If you meant multi-processor systems, the most likely reason, with BOINC
for not using all the processors is that it has not been made aware of them.