The other nice feature of the ELC is that a player can be signed to one after he is drafted, but if he is 18 or 19 and doesn't play more than 9 NHL games in a given year, his contract 'slides' to the next year and he will still have 3 years remaining on it.
Smart NHL fans have made the calculation themselves - well, if you have a very high draft pick, like the Oilers seem to every year, why would you put him in the NHL right away? Your team is terrible - adding that player adds next to nothing to your playoff chances. Plus, you lose a year off his valuable ELC.
If you held that player out until he was 20, you'd get more bang for your buck - the kid figures to be better at 20 than he is at 18, and you still get him on the same cheap three year contract.