It's not technology, it's the the nature of the company itself. I know all websites want to keep you around, but the difference between using gmail and yahoo mail is remarkable.
I just went to mail.google.com, and I see my email. Now I'm going to yahoo mail… there's a big ad for life insurance next to my messages. Above them is a slightly larger link, intended to look like an email messages, for "rich dad, poor dad." Actually, something interesting has happened lately… the link expands in size about 3 seconds after the page has loaded. I'm starting to wonder if yahoo has noticed that if they do this, they will expand the ad just at the moment that the users wanting to check their email click on the first, top message, leading them instead to click on an ad, increasing "click through".
I'll admit, I'm leaning toward the tinfoil hat side of suspicious with that last one, but there's a reason I've started to think this way. Go to yahoo's front page, and it's click bait city. It seems like yahoo will do everything in its power to make sure I don't do what it is I came to the site to do, run a web search, or check my email.
In short, it appears to me that yahoo is still a media company, following the portal model, trying to get people to stay on the site for reasons that had nothing to do with why they came to the site. And by click bait, I mean titles like "the one medical test you shouldn't skip!", or "the 5 signs that he's manipulating you". In short, I'm starting to wonder if having a yahoo email account is actually a risk to my ability to concentrate.
Maybe yahoo is becoming more of an engineering/technology company, maybe not. If Mayer can make this happen, that'd be quite an accomplishment. There's definitely a huge legacy of specifically intending go not be an engineering company.