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According to Wikipedia, there was only one Falcon 9 booster landing on ground-pad in 2021, all other 30 launches at least attempted to land on a drone-ship.

Comparatively, 2020 and (so far) 2022 both had about five times more attempts on drone-ship than ground-pad, rather than 30 times more.

Why was 2021 different?

Booster landing graph from Wikipedia

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Whenever possible, SpaceX prefers Return To Launch Site (RTLS) landings over Autonomous Spaceport Droneship (ASDS) landings, since they get the booster back immediately instead of having to wait for a 3-day sea voyage (with all the associated dangers), marine assets are expensive to operate, and sea water is aggressively corrosive.

Also, whenever possible, SpaceX prefers to launch from Florida instead of Vandenberg, since they have many more boosters, many more facilities, many more marine assets, and much more experience with processing in Florida than they have at Vandenberg.

The only reason to not launch from Florida is the same as the only reason to land on an ASDS: you don't have enough performance to do anything else.

So, the quite simplistic answer is: in 2021 there were simply (almost) no customers with light payloads to low orbits.

Until 2020, SpaceX was still flying Dragon v1 for the Commercial Resupply (CRS) missions to the ISS, which was light enough for an RTLS landing, however, in the middle of 2020, SpaceX switched to Cargo Dragon v2 which is too heavy for an RTLS landing. That alone accounts for a difference per year between the years before and after 2020 of 2–4 CRS missions which used to be RTLS landings in the past and are now ASDS landings.

The Transporter smallsat rideshare missions are either RTLS or ASDS, depending on how many and how heavy smallsats are flying on each mission. In 2021, there happened to be only one Transporter mission, likely because of CoViD-19, whereas in 2022, there were already three (of which two were RTLS landings), with a fourth one planned for November.

This year, there also happened to be several National Security launches of smaller payloads that allowed an RTLS landing. The reason why those launches all happened to be bunched up this year are not fully known, but we can take two guesses: they were delayed by CoViD-19 or they were made necessary by the Russian invasion of Ukraine (or a combination of both).

So, in 2017–2020, a significant portion of the RTLS landings were CRS missions, which however now have to be ASDS landings. The numbers for 2018 and 2019 are further inflated due to three Falcon Heavy launches resulting in 6 RTLS landings for the side boosters. (There have been no Falcon Heavy launches since.)

In 2022, we saw two Transporter launches (which didn't exist prior to 2021 and only became more regular in 2022) and three lightweight reconnaissance satellites.

In contrast, in 2021, only 7 of the 31 launches were not Starlink or Dragon v2 launches (none of which can RTLS). Of those 7, three were interplanetary, which requires more performance, two were very heavy communications satellites in geostationary orbit, and one was a GPS satellite, which are also pretty heavy.

Or, in short: 2021 just happened to be a year with no light satellites.

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    $\begingroup$ Awesome answer. It might be worth adding a glossary of the abbreviations used here (e.g. RTLS, ASDS, CRS, etc). $\endgroup$ Jul 27, 2022 at 9:43
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    $\begingroup$ Thanks for the tip. I expanded all uncommon TLAs and ETLAs on first-time use. $\endgroup$ Jul 27, 2022 at 12:48

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