Wouldn't want to put words in his mouth, but guestimating ...
I'm presuming this is for Partimus.org. I think this is in part some history/momentum ... basically I think it's gone approximately like: Lubuntu was earlier chosen, as it being in the *buntu family, relatively well supported (community 'n all that), pretty popular/common, etc., and Lubuntu, for being a more lightweight install, less consuming of resources (particularly noteworthy for older hardware) - and where the user base pretty much needed/"demanded"/required GUI environment and such. Also, smaller ISO to install from - I believe among the *buntu 14.04[.x], Lubuntu was the last desktop one to fit on CD (<=720,000 KiB) - some older hardware having CD, but not DVD drives, and possibly not supporting boot from USB (at least not directly). I think there's also fair bit of "momentum": procedures, configuration, documentation, etc., built up around Lubuntu 14.04[.x] for use by Partimus.
Anyway, that's my guestimate as to why Lubuntu 14.04[.x]. Why 64 bit? I have no idea - I'd think 32 bit would be preferred, as it will run on more lower end / older hardware, and consume a bit less memory.
Ah, but the *scary* bit I notice ... Lubuntu 14.04[.x] went EOL 2017-04 - so *no* security support, bug fixes, etc.
I always thought Debian Edu / Skolelinux would be a most excellent fit for Partimus.org, but thus far appears Partimus.org has gone in slightly different direction(s). Debian Edu / Skolelinux is also a Debian Pure Blend. And though Canonical naming convention is a bit different, Lubuntu shares similar relationship with Ubuntu - same repositories, "just" different selection of packages - and modestly different configuration. However, unfortunately, in the Lubuntu case, the support is different and ends earlier than that of Ubuntu. Maybe a Lubuntu (semi-)expert can fill in the details a bit better, but those Lubuntu specific packages (and/or the repository they come from?) - the support falls off sooner than that of Ubuntu. Debian also has "supported", and, after that (with some limitations), LTS support - but the Debian LTS model and what it does and doesn't cover is somewhat different than the Ubuntu model (and any other *buntus).
references/excerpts: https://www.wiki.balug.org/wiki/doku.php?id=balug:cds_and_images_etc https://en.wikipedia.org/wiki/Lubuntu#Releases http://www.partimus.org/ http://www.skolelinux.org/ https://blends.debian.org/blends/ch02.html#Blends
From: "Rick Moen" rick@linuxmafia.com Subject: Re: [BALUG-Talk] Flash drives for Lubuntu 14.04 4 bit Date: Fri, 28 Jul 2017 11:15:16 -0700
Quoting Christian Einfeldt (einfeldt@gmail.com):
Does anyone have any recommendations / precautions about which flash drives to use or to avoid for Lubuntu 14.04?
Why specifically Lubuntu, and why specifically 14.04?