Not only a question of technology being slower, but reducing benefits that also makes the need for the jump to be bigger to be noticeable and worthy.
Two issues that go hand in hand. Five years used to be more than enough to give us a massive leap in technology. We went from the SNES in 1991 to the N64 in 1996. The GameCube's GPU was rated at least 47 times more powerful than the N64's. The PS3 was a comparably large increase over the PS2. Meanwhile, the PS4 & XBO weren't even a full order of magnitude more powerful than the PS3 & 360 despite a seven year gap. Honestly, the PS4 Pro and Xbox One X are how powerful the PS4 & XBO should have been at launch, but couldn't because they'd be prohibitively expensive in 2013. That was a ten-year gap between the PS3 and the PS4 Pro.
It's taking longer to get the kind of power increases we used to see in previous generations. And then there's the diminishing returns. While there's still a ton of advancement that can be done with game graphics, the leap from the 360 to X1X doesn't yield the same qualitative increase in graphics that we saw in the jump from the PS2 & Xbox to the PS3 & 360, which in turn wasn't quite the jump we saw from the PS1 & N64 to the PS2 & GameCube. It takes a significant quantitative increase in specs to yield a quantitative increase sufficient to justify the upgrade to a new generation. This is especially the case today with the push for 4K and and 60 fps as standards, as those things come at a price. If we stuck with 1080p30 as the standard, we'd see better graphical results.
The slowdown in tech advancement combined with diminishing returns is why last generation was so long. And that's why this generation needs to be at least as long. The PS4 & XBO still have the potential to generate steady sales well into 2019. We've seen YoY growth for the XBO thanks to the cut to $230, and, especially given how well the PS4 did in November, Sony could also generate significant growth for it as well by cutting the PS4 to at least $250. I don't want to see next-gen rushed, and I absolutely do not mind waiting until 2021 for next-gen consoles. I also wouldn't mind paying $500 at launch, either. Assuming 2% annual inflation, by Nov. 2021 the PS4's inflation adjusted launch price will be upwards of $460, and it was pretty damn reasonable price at the time (on par with the PS2 & Xbox, not too far ahead of the SNES & Genesis). I think the market will be ready for a $500 launch price by 2021, especially if the technological leap is worth it.