Asana blogged this year about 50k organisations, 6 quarters accelerating growth [1].
Back in 2015 they blogged mentioning the heating up 'enterprise collaboration market'. There's countless office tools, diagramming tools, collaboration tools. Even in software there's more CI systems than I can count.
My guess is there's a big market which the VCs think is accessible to Asana in a way it's fundamentally not to a competitor. Combined with big growth it makes sense.
[1] https://blog.asana.com/2018/09/asana-company-updates-2018/