I think the fact that 20+ Conservative MPs and officials are being investigated for electoral fraud (possibly a criminal offence - presumably if proven to be a deliberate act), is likely to have had some influence. Better to go to the country relatively untarnished so far as the law is concerned, and from a perceived position of strength than to risk the chance of proceedings being brought and potential voters being alienated.
Also, of course, if the Conservatives win she will claim that gives the party the right to be left alone to decide the terms of Brexit without people claiming they haven't got a mandate.