I suspect that Mossack Fonseca avoids taking on American clients to skirt the federal Foreign Accounting Tax Compliance Act (FATCA). FATCA gives the IRS many powerful tools to pressure foreign banks and financial institutions to disclose information about American citizens in order to ensure that those citizens are reporting all of their income. This frequently involves disclosures of non-Americans' finances, because stashing income in a corporation with foreign citizenship is an easy way to evade income tax.
Many, many non-American financial institutions categorically refuse to provide services to American citizens in order to avoid IRS scrutiny of their books and to protect the privacy of their non-American clients. I expect that, before offshoring dark assets, the likes of Vladimir Putin and Xi Jiping would select a management firm that poses minimal risk of disclosure to US regulators and thus a firm that has no American clients.
Meaning, a firm providing these kinds of services can probably only serve either US citizens OR the government officials of America's geopolitical rivals, but not both. The presence of those officials does not indicate that Americans aren't engaged in similar activities, just that they are probably serviced by other firms and so won't be implicated in this particular leak.
Otherwise, there are three possible explanations for the lack of Americans that I can think of.
First, later releases may reveal US citizens in the client list, rendering this discussion moot. I have no idea why the journalists would wait to disclose information about Americans, but we might get some idea about that once further details emerge.
Second, the journalists may be protecting their own nations' interests, as posited by this article. Personally I find that unlikely given that the ICIJ has a track record of integrity and releasing information that is damaging to US persons. Plus, I suspect if they were protecting certain parties, they wouldn't protect an entire nation's citizenry but would instead selectively exclude key figures from the releases.
Third, it is entirely possible the party that released the data edited it before hand to exclude Americans and Germans. I think that may be the explanation if it turns out that this is a joint NSA-BND operation posing as an anonymous, solo whistle-blower. I could see a basic inter-agency operational agreement stipulating that the leaked information should not implicate anyone from the home nations of the participating agencies to avoid thorny political issues arising from intelligence agency activities that interfere with domestic politics.
Regardless, I personally see my original FATCA theory as the most probable explanation. But we can't know the true explanation given the current released information and ultimately we may never know the real truth.