Were there large import tariffs or something making it prohibitively expensive to get Apple computers from the US, or did Apple not market the machines internationally?
Countries such as the USSR and Brazil simply made their own Apple clones. I'd suspect Western Europe did not do so because of legal ramifications.
No it wasn't down to tariffs. Keep in mind Commodore did immensely well in Europe despite being a US company (though in name from some point in time it was a Bahamas corporation for tax reasons)
It was many factors I think.
Apple indeed was not set up to handle international sales well, while Commodore had a huge international channel owing to a much longer history (the progenitor of Commodore Business Machines was founded in 1954).
But all indications are that Commodore (and probably Tandy) outsold Apple substantially in the late 70's in the US too. At that point Apple got the "hacker" market, Commodore focused on the business market.
It wasn't until the VIC 20 that Commodore started aggressively going after the consumer market, but the VIC 20 was a low end product, and it was probably first with the C64 that Commodore had a product going after some of the same market as the Apple II.
At this point Apple increasingly was the expensive choice, and the European market was apparently a lot more price sensitive. On top of that Apple had much more aggressively pursued schools in the US, and so I guess US parents might have seen the Apple II as a more "serious" choice at a time when Commodore was increasingly getting associated with games.
Commodore thoroughly trounced Apple in worldwide sales of 8-bit machines and probably for quite some time in 16 bit models from soon after the introduction of the Amiga, but more and more of their output went to Europe for these reasons (eventually quite a lot of Commodores machines were also manufactured in Germany) but also because Tramiel quickly realized that the European markets were higher margins, and so any if supply couldn't be ramped up quick enough for certain models, Europe got priority.
Commodore also burned itself in the US, and its US dealer network, through extreme price war with Texas Instruments and Tandy (pushing things so far that TI pulled out of the market and posted massive losses, but also massively hurting Commodore's profitability - likely a major factor when Jack Tramiel was ousted) and the damage it did to its good will with its US dealer network probably shouldn't be under estimated.
(If you're interested in the Commodore/non-Apple view of the early home computer market, "Commodore: A company on the edge" by Brian Bagnall is a fascinating read)
Countries such as the USSR and Brazil simply made their own Apple clones. I'd suspect Western Europe did not do so because of legal ramifications.