Why is reaper rushing used less frequently nowadays?

As you reach higher levels of play, if scouted properly, the defender will have enough units that will hold off the reapers from damaging their harvester lines. Granted if they don't catch it, it can do massive damage to their economy as the reapers have a +damage versus light units (which all harvesters are).

In gold or below leagues, the reaper rushes can still do a lot of damage, but if you watch any of the higher level matches (most can be found on HDStarcraft or HuskyStarcraft's youtube channel) most high level players will use the reaper to scout and try to harass (they might get 2 or 3 harvester kills), but it doesn't do much else more than that except the very rare occasions with good micro.

It's also very situational as well, against early zerg yes you can kite zerglings with no speed, but once they have the speed boost, reapers are surrounded and chewed through. Against protoss, you can kite the first zealot they put out, but by the time the zealot is dead, a stalker will almost be done and that stalker will kill the reaper. Against other terrans, if you go for their SCVs, the marine will kill you.

Personally I sometimes use it against either zerg or protoss, never against other terrans.

Reaper rushes require a significant investment as opposed to reaper harassment. If you are going to use a reaper to harass, you'll be investing enough for 1-3 at the most to use as scouting or to pick off a few probes. The mineral/gas investment for a quick scout/harass with units that can ignore cliffs usually pays for itself before the units are countered and destroyed as you will not be scouted and countered in time if you push with a small number of quick units (especially if they aare built at the same time in different barracks).

If you're committing to a rush, however, you're committing to more reapers as well as their speed upgrade. The problem with committing to a ton of reapers in higher level play is timing. By the time you have enough reapers out to make a rush effective, any good higher-level player will have scouted your build and had plenty of time to build effective counter units. In addition, the units they will counter with will be effective in many more scenarios than your reapers will be. Once your reaper rush is countered, the most you can hope to do is use them for random scouting. In the meantime, your opponent will have a collection of units with increased military value.

Reaper rushes work in lower-level play because players tend to mass units blindly without scouting, don't establish counter builds, and don't defend their mineral line. Against opponents such as this, a reaper rush can be devastating.

I feel like reaper rushes were also popularized when the game was very new because of the novice maps used for placement matches. The reapers, with their ability to jump up cliffs, were a very good way to jump past the rocks, into the opponents base, and rip through a noob's economy.

I figured once the initial surge of placement matches were finished, and blizzard nerfed their required tech tree, the fad died down.