I suspect the reason is at least partially financial - it's probably cheaper now to just let the full order of episodes of a new show air than to pay for extra episodes of a backup substitute. "Minority Report," for instance, only had a had a seven episode order and will be replaced by "Lucifer" in December. With so many spots on the schedule becoming less competitive, there are a lot of places to stash an ailing show without pulling it completely. Also, executives are clearly nervous about the uncertainties of the rapidly changing television ratings and scheduling models, and are willing to give struggling shows more time to improve. The internet and DVR viewing have severely cut into traditional live viewing, such that the Nielsen ratings model was adjusted a few years ago to take into account audiences that watch a show via DVR within a certain timeframe, usually Live + 3 and Live + 7. It's what kept more niche shows like "The Office" and "Fringe" on the air. Now, with live audiences dropping even further, it's even harder to determine what might be worth keeping around.
Audiences and content creators benefit to some extent, as new shows stay on the air longer and get more chances to connect with viewers. Everyone can name a few ambitious, interesting shows that were cancelled because they couldn't deliver ratings quickly. I used to get so exasperated when weird, cool little genre shows like Bryan Fuller's "Wonderfalls" or Tim Minear's "Drive" would only last four or five episodes on FOX (it was almost always FOX) before vanishing into obscurity. Last year's "Selfie" probably would have survived a little longer this year on TV. The downside, however, is that the shows that clearly aren't working will end up hanging around long after they should have gotten the boot. Remember when "No Ordinary Family" got a full season order, but everyone stopped watching after three weeks? Or the whole fiasco with the "Michael J. Fox Show"?
There's no telling if this is a trend that will continue, or if this season Is just a fluke, but the lack of cancellations seems to be the latest symptom of network television having to completely overhaul how they operate to keep up with its internet and cable competitors. We've seen episode orders shrink, pilot season in disarray, shows skipping from platform to platform, and executives second-guessing everything as audiences continue to steadily migrate away from live television. There have been some interesting experiments - ordering straight to series, FOX declaring an end to pilots outright, and increasingly aggressive advertising tactics. You can definitely see Netflix's model having some influence, with the newly announced "Star Trek" series being earmarked for CBS's new proprietary streaming service, and NBC's releasing all of "Aquarius" online at once over the summer.
At the moment, the only shows that have been cancelled in 2015 have been the ones that have already had more than a fair shot - Syfy's "Defiance," CBS's summer series "Extant," and bunch of one-season cable wonders. We also know that some shows' current or upcoming seasons will be their last, "American Idol" and "Person of Interest" among them. I'm sure the bulk of the new shows from this season will be joining them, but we'll have to wait a while longer to find out which ones.