Poor leadership, and failure to attract talent. They didn't have the same culture pull as Google/Facebook/Apple in the latest tech bubble. And why work for Yahoo if you could do the same thing with VCs funneling you money? I think one of the most telling things is the lack of OSS from Yahoo. Their top repo on github has been effectively dead for around a year [2]. Yahoo hasn't been a tech company is almost a decade, and it shows in their products. Not even their numerous acquihires have netted them anything.
EDIT: Looking at a timeline of Yahoo, they haven't launched a major product since 2006 (10 years). All new products have been bought [3]. Imagine if Google's last product was Docs. No Android, no Doubleclick, no Street View, no autocomplete in search, no Chrome.
[1] https://en.wikipedia.org/wiki/List_of_Yahoo!-owned_sites_and...
[2] https://github.com/search?o=desc&q=user%3Ayahoo&ref=searchre...