Investors (including YC) prefer to invest in teams focused on one specific product. Every successful company will end up competing against other companies with similar products, so ones that are more focused are more likely to win.
Investors already have their eggs in many baskets, so additional diversification within a given investment isn't attractive to them.
Bingo. There has to be some correlation among the products else you run into competition on all fronts. On top of this - the competition you will run into are often solely focused on that product.
Investors already have their eggs in many baskets, so additional diversification within a given investment isn't attractive to them.