Why not shorten times for elite MMR players by making the # of players they can play with bigger?

With the issue of huge queues being much longer for higher MMRs than it is for lower, why not broaden the spectrum of players that they can play with a little, or have it broaden over the time of the queue?

For instance: A Master or Challenger player is usually queued with the absolute best of the best. These are the elite and there isn't exactly that many of them. Wouldn't it be easier to expand the people they play with into lower Diamond as the queue dragged on in order to give a high quality match without the wait. Could the same be done in Diamond, where it expands into Challenger and at the lowest Platinum II?

On the other side of the coin, is the MMR difference really so high that these elite players can't queue up with those of lower elo?