Why has indexed shifting displaced traditional friction shifting?

Indexed shifting keeps the rider's hands on the handlebars. Indexed shifting also correlates with a move to integrated shifters for road bikes, so the hand position doesn't move, at least on the hoods and drops.

I can imagine some sly race tactic in the 80s with a rider timing their breakaway to coincide with a gear change by other riders in the group. Having your hands on the brifters means your hands are not ~30 cm away from the bars, and you're faster to respond.

---

Modern bikes have 10+ gears across the back, that's a lot more positions and a friction lever would have narrower "good" spots for each gear making accurate changes slightly more difficult.

With that many gears, it is more common to change up/down to a slightly different gear for comfort or power. If the shifter is further away and needs a more coordinated movement, you're less likely to do it. An older 5/6/7 speed bike might have 3~4 tooth jumps between some gears, so the cadence can change a lot compared to a single tooth jump, which is more common when there are more gear cogs across the cassette.

In short - fewer gears meant fewer shifts, and riders might have been pedalling slightly outside their preferred cadence to maintain the bunch's pace.

---

In addition, keeping your hand position still would be more aero - leaning down to downtube shifters will move the whole upper body. Even stem shifters move the whole arm, changing airflow.

---

You can doubtless get friction-shifters still but they are definitely less common. Properly tuned indexed shifters work very well, and are a gateway to fully electronic shifting which is functionally identical for the rider.

---

Summary moving the shifting to the brake lever forced the use of indexed shifting.

^{this answer applies equally well to pod/thumb/trigger shifters on MTBs, but there's less "race craft" in MTB and more riding technique, so reacting quickly to changes in the peloton is less important.
Also, MTBs never really had downtube or stem shifters, their friction shifters were normally bar-mounted levers so were closer to the grips.}

Index shifting I've found to be somewhat unreliable. shifting down may go smooth but going up again will often not work unless you jump a gear or vice versa. No amount of adjusting will cure the problem. The front shifters aren't so bad and oft times the front is friction anyway controlled anyway.

One point that seemed to be missed in the answers so far (though I'll admit I didn't read every word) is that indexed shifters are only a small part of indexed shifting.

It used to be that you needed friction shifters because, in order to shift to a larger sprocket, you had to "over-shift" substantially -- push the lever beyond the point where it would eventually end up, then move it back once the chain had begun to move. Because of this, indexed shifting was essentially impossible, even though it was no doubt a sought-after goal of many inventors.

What changed was the chain and sprockets, with carefully-engineered profiles that would cause the chain to "climb" the sprocket if it was pushed ever so slightly in that direction. This invention made indexed shifting possible, and largely eliminated shifting as a major roadblock in the way of a "mass market" for bikes.

But, rather incidentally, the change to chain and sprocket profiles made possible shifting under load, something that was largely impossible before. While many of us would willingly give up indexed shifting (I miss the sensation of the lever that allowed me to inherently know what gear I am in), how many would be happy to give up being able to shift under load?

What I see is they're forcing bicycles to be like the automobile market; "old is bad" and "replace every 3 years (or sooner if WE TELL YOU TO)". Anything they can do to keep selling parts and bikes is "Good" for the "Economy" (AKA THEM) I just damaged the frame on my 16 year old Specialized Hardrock. The indexed "collar grip" shifters had cracked and failed in the first 6 months, fitted on the thumb lever friction shifters from my previous bike, and replaced parts as they wore out over the years. My new bike is in the garage half disassembled as I refit it, replace the front shocks with a solid fork, friction shifters, 'long enough' seat post on order, and installing my double layer thorn stripping. My real problem is I'm running out of parts as I repair other peoples bikes. Dumping the indexed shifters usually fixes them. Thorn strips and ooze filled tubes fixes their other problems. Indexed shifters are fine if you want to take the time to tune them, and if you ride frequently; tuning them twice a month. I am considering manufacturing my own friction shifters during slack time at work, but it's not a cheap way to do it.

Hehe- I'm a fossil who still uses downtube friction shifters on my 1972-vintage Euro roadster.
But I do admit that as I get even yet older the allure of those nifty "brifters" is there.

Once learned, they are easy to use, forgiving of adjustment, and accommodate a wide variety of gear clusters with no problems. I took my originally 5-speed rear end up to an 8-speed with no change at all to the shifters.

As noted, indexed shifting does have its problems as well. I see many kids here at the university with multi-speed mountain bikes who get off and push the bike up hills, or struggling with a way-too-high gear. I have stopped a few and asked why, and the answer is usually: "It's too hard." or more likely, "It shifted OK when I got it, but now it doesn't work." Kind of sad. A well-adjusted gear-train is a joy to use, gears effortlessly leaping from cog to cog. Usually, after an initial adjustment after new-cable stretch, shifters will stay in adjustment for a long time. I'm surprised that riders who are not maintenance-prone don't use the hub-shifters more.