"Simple" explanation is this:<p>- May isn't going to lose to Corbyn<p>- May has a slim majority, meaning she has to negotiate with people within the Conservative Party who might not agree with her on the Brexit terms.<p>- She'll likely get a larger majority, so it makes it easier to deal with the rebels.<p>- For Corbyn, it's interesting. He might be facing another leadership challenge if polls keep looking crap.<p>- He wants to have an actual attempt at a GE, where he can make his case to the public.<p>- He won the leadership challenge last year, so he knows his fans are at least loyal for now.<p>- Why does Corbyn's opinion matter? Well back when Cameron did a deal with Clegg, they did a fixed terms act, meaning elections had to happen every 5 years. This one is within 5 years of the previous, so you actually need a two-thirds majority in Parliament to trigger it.