What caused Stallman etc. to jump on the Thinkpad ?
I know this is a opinion-based question, but this is something that you can answer just by crawling through Stallman's site:
How I do my computing
I use a Thinkpad X60 computer, in which the FSF installed a free initialization program (libreboot) and a free operating system (Trisquel GNU/Linux.) This is the first computer model ever to be sold commercially with a free initialization program and a free operating system, and thus the first computer product the FSF could endorse. (It was not sold that way by Lenovo, however.)
Before that, I used the Lemote Yeeloong for several years. At the time, it was the only laptop one could buy that could run a free initialization program and a free operating system. But it was never sold with a free operating system.
Before that, I used an OLPC for some weeks. I stopped because the OLPC project decided to make their machine support Windows, so I did not want to appear to endorse it. The OLPC uses a nonfree firmware blob for the WiFi, so I could not use the internal WiFi device. No big problem, I used an external one.
No need to be a genious to search on other person website...
Also, there are some implicit things on this notebook that may turn it into a better option for those looking for privacy like the absense of Intel AMT/vPro feature that could be used to breaks one's digital privacy.
Other question is that this notebook does not depend on non-free blobs(as you can see, it runs Trisquel GNU/Linux out of the box)
FSF has also created a product from it, reselling old Thinkpads with libreboot preinstalled(X60 still selling, X200 discontinued):
The Gluglug X60 laptops are refurbished models of Lenovo's ThinkPad®† X60. Gluglug has updated the computer by adding a modern wifi chipset, replacing the proprietary BIOS with a free software boot system made by modifying Coreboot to remove all proprietary microcode and proprietary blobs, and replacing the proprietary Microsoft Windows OS with the FSF-endorsed, Trisquel GNU/Linux OS.
However, this only answers RMS needs. The rest of the community pretty much uses Thinkpads cause they are known to have good compatibility on Opensource OSs. If you take a look at OpenBSD lists, there is a pretty decent number of developers that uses Thinkpads, and even modern Carbon X1 ones.
If you browse at Ubuntu forums, you will see that most of the problems are solved by firmware install or Kernel updates.
These are two separate questions.
Obviously RMS is going to endorse adapting a laptop with a BIOS which is free software and has no closed-source blobs. So that answers that.
The first question is why people can and do work on coreboot (which accepts some blobs), and then libreboot, on these old Thinkpad models in the first place.
AUIU, the main Thinkpad series are arguably the most premium laptops. Barring gaming hardware. And ignoring Macs, which are an uphill battle to get anything third-party to support on, due to the amount of custom work. They're thickly built (they generally last). The X-series were comfortably small before business "ultrabooks" (Mac Air style thin form factor).
For whatever reason, they've stuck with premium Intel chips instead of competitors. And going back at least to Matthew Garret's first presentation on the subject, if you want Linux to work on your laptop, looking for Intel is a good rule of thumb. In particular for Wifi. Intel write upstream drivers.
(More recently, Dell's Project Sputnik has made them more attractive for some Linux users. Dell also have Intel wireless as an option).
Cheap consumer laptops also proliferate different models. If you want to hack firmware, you want one popular model you can work on.
Lastly - notice these are X60's and X200's, which are old. The commercial libreboot products are "refurbished", second-hand machines. Thinkpads are popular in business, they last, and then they get sold on. (Libiquity claim some new parts, but without specifying which, we may as well assume they mean the stickers). So we have a popular model, which was around long enough for people to crack the blob problem, and which is still available to use, repaying that initial investment.