Some valid points already made, but I think money has a lot to do with it too. Look at the figures for example:
Base fee for group stage: €1,300,000 (Europa League) - €8,600,000 (Champions League)
Group match victory: €200,000 - €1,000,000
Group match draw: €100,000 - €500,000
Group winners: €400,000
Group runners-up: €200,000
Round of 32: €200,000
Round of 16: €350,000 - €3,500,000
Quarter-finals: €450,000 - €3,900,000
Semi-finals: €1,000,000 - €4,900,000
Losing finalist: €2,500,000 - €6,500,000
Winners: €5,000,000 - €10,500,000
So a club like Newcastle would have made what, £4/5m from the Europa League this season? Had they made the Champions League they would have made double that just by qualifying, even if they lost every game. It just isn't worth it until the semi finals. This isn't even taking into account the vast riches of the Premier League. Last season, Wolves earnt almost £40m even though they finished bottom.
Everyone in the Europa League who qualifies from England is either fighting for the Champions League spots, or to avoid relegation. Both of which will obviously take priority as there is so much more at stake from a financial point of view.