Reason #1: It overloads them mentally. It was hard enough to wrap their minds around Mastodon not being the one website they thought it is.
Reason #2: They got so used to their nice and cosy and fluffy and friendly woolly mammoth being the entire Fediverse, and everything they ever interact with being Mastodon, that everything they (might) have to interact with that is not Mastodon is too much of a disturbance. There being something else in the Fediverse other than Mastodon simply feels too wrong.
No. You can’t crosspost to two or more Lemmy communities at once. AFAICS, that’s fully deliberate and intentional by design to keep people from spamming Lemmy with mass-crossposts.
What it fixes is trouble with crossposting to Lemmy, Friendica/Hubzilla/(streams) and Guppe groups. You can’t mention these in any order you like. You always have to begin with one Lemmy community. If a Lemmy community is not mentioned first, it will be ignored, no matter what is mentioned first.
Also, apparently, mentioning Guppe groups before a Friendica group, a Hubzilla forum or a (streams) group doesn’t work either.