Because they are the clear market leader in the category, have been for a long time, only recently falling behind Microsoft. (within the last 2 years of their 20 years as a company.) Give it a year or two to integrate into Salesforce and they will be neck and neck. This is just as much a bailout to Tableau to save them from being dwarfed by Microsoft.
You make it sound like the ticker symbol justifies a $15.7B price tag...