personally i don't think it has much to do with echolink or IRLP...since
there aren't a whole lot of HF privvallaged folks on there anyway.
Its probably got more to do with the fact that most people in our day and
age don't have as much time for persuing their radio hobby as they once did.
Also, with the american operators becoming more and more restricted as to
antennas in housing developments, with hams who previously operated HF, now
having to stick to 2M and 70CM because they can only use a mobile set up.
I think echolink and IRLP are pretty much the least of the issues out there
that are causing us to hear less and less folks on the HF bands.
Lets not forget that many many hams simply aren't bothering to get on the
air because they do not like to deal with unstable conditions and are
waiting for conditions to get more stable, if not necesarily better.
the threat of BPL has also thrown a shadow over many, and their plans for
antennas and so on have been put on hold until plans are finalized one way
or the other for BPL projects...no point in putting in an expensive antenna
system when it will prove useless when BPL goes through in your area right?
I think another big problem, especially right now, is that there are less
and less US hams that are getting HF privallages because the regulations are
still quite stringent.  You will hear many many more hams from Canada and
other countries who have relaxed the code requirements on the HF bands now
compared with a year ago.
These are surprisingly very good operators and I personally have seen
nothing but good come out of relaxing the code requirement here.
73
Colin, V A6BKX