Various things.
5,821 seems like a good attendance for a midweek evening game with a kick-off time of 5:00pm. Especially as the game was shown live on BBC2.
The deal with ESPN doesn't help the reach for WSL games as ESPN is very much a minority cable channel in Britain (in fact, will Disney even bother continuing without Premiership football rights?), and even with that the WSL games are treated as schedule filler, shoved around into wherever they have a hole. But I'm not sure any of the other broadcasters would do more, without coercion. c.f. that 5pm kick-off time for the England-Croatia match, presumably purely for the convenience of the BBC. On the other hand, the WSL is an attempt at a professional, or at least semi-pro, league. So giving their games away for free to a more mainstream channel when they have a cash offer from elsewhere isn't really practicle. The league can't afford to play a long game, slowly building itself up. It needs all the funding it can get.
Personally, I think the BBC should be covering the WSL and paying to do so. Not as a commercial decision, but in it's role as a public service broadcaster. The gender imbalance in the sport they cover ought to embarass them.
I also think the FA should be bundling the Women's team in with the Men's and putting the two FA Cups together, with more coercive restrictions on the chosen broadcasters to give the various elements as close to equal prominence as they can be arm-twisted into. At the very least, every England Women's home competitive match screened live in a prime time slot on their main channel, if not neccesarily scheduled against other football. And if that makes football internationals less attractive for ITV to buy, well then everyone benefits.