Why? Because once EOS begins it's crowdsale on Ethereum the amount of ETH which could be locked up can be massive and for a long time. They intend to have the sale go on for a year using the same method used in AGS with some modifications. In my experience as a participant in AGS, I recall it being the most successful crowdsale ever at the time. From a psychological perspective every day was a brand new day and new chance to capture a nice sized stake.
If you are reading this right now and intend to get in on the EOS sale, you should buy your ETH before the obvious pump of ETH occurs. This is one less reason to sell ETH and another lock up of potentially thousands of ETH for years. It is possible we can see $500 ETH this summer and $1000 in 2017 just due to buy pressure from crowdsales. Tezos also has a crowdsale this month and will accept ETH as well which means potentially even more pressure.
Of course if you think ETH could crash soon, maybe now isn't the best time and it is better to wait. Also maybe you hold something like Bitcoin which is also going up at the same time? Currently though we are at ATH territory and I don't see anyone being able to dump the price once the EOS crowdsale begins.