Here's some more data points. According to AngelList
https://angel.co/online-dating ~38,000 investors
https://angel.co/cryptocurrency-2 ~989 investors
https://angel.co/travel-tourism ~1,082 investors
Yet the market for Travel is $300b (online) and the dating one is $2b (online).
How does that constitute as investors not wanting to invest into dating startups when there's 38x as many investors (by absolute numbers) as travel and yet the market is 150th the size? Seems like it's quite the opposite, no?
If anything the talking point here should be "why are so many people investing in dating apps when their returns aren't great?"
It'd be also helpful to define what you consider "mainstream" before drawing a conclusion...
[0] - http://www.wikinvest.com/stock/Expedia_%28EXPE%29/Data/EBITD...
Travel planning software: The most common bad startup idea
https://news.ycombinator.com/item?id=8419658
So maybe it is not surprising to find few investors in it?