Series are perceived as easy to market.
Why is that though? I mean, svb gives a good reason, but it wasn't too long ago that an aspiring author would be told "Go write a good single novel that sells well, then we'll look at your series." Now there does seem to be a trend towards series. Even though the market is more competitive than ever, and the risk of backing a new author is greater than ever, publishers do seem willing to throw in on a series.
I will say that I think if you have a few books written already, they are more likely to run with it. Even novel submission sizes are growing, which is another potential money sink for a publisher. Baen (who does the Ringo's various series and Weber's Honor Harrington series) asks for a
minimum of 100k words, up to about 130k. Wow.