Why not <= then? I'd expect both < and = to be available.

Probably because '<=' is very easily read as "less or equal"? (unless you are joking, of course)

But why? := is perfectly understandable and established notation for definition. You don't get closer assignment without resorting to esoteric notation?

That is indeed the case in Smalltalk.

The Xerox keyboards had the support for ⇐, with its workstation OSes being all graphical based.

When Smalltalk grew beyond Xerox PARC walls, ⇐ turned into :=

You can see this in the original Xerox PARC documents and books for Smalltalk.

In some of the draft versions of ASCII the positions currently taken by underscore and caret were left arrow and up arrow respectively. As late as 1985 I used terminals (LanparScope) the supported the older draft.

All Commodore machines also uses the older ASCII standards, (in addition to filling out the full 8 bit range with various graphics). It’s usually referred to as “PETSCII”, from the original Commodore “PET” series, but PETSCII was also used in the vastly more popular Commodore 64 (and 128) home computers.