The primary reason is one of competence.
The Russians have a series of pretty darn good rockets.
The RD-171, a 4 engine bell, one turbopump engine (used on Zenit first stage) is an impressive Kerosene/LOX engine. It is a competitor for the F-1 used on the Saturn V in terms of capability.
The RD-180 is a 2 engine bell, smaller turbopump version of the RD-171 and as Atlas V has shown, a pretty darn good engine.
The RD-193/181 is the 1 engine bell version of the same. Orbital will be using 2 of these to replace the NK-33s on the Antares booster.
Heck, even the NK-33 built in the 1960s and 1970s is a darn good engine for today, which is why Orbital started with it.
The Russians do not do as well with LH2/LOX engines; there the US is world class.
Thus there are a number of useful engines made by the Russians that they are happy to export.
On the European side, there is the Vulcain engine used on the Ariane V, and not much else. The SRBs they use on Ariane V boosters and on the Vega light launcher are mostly equivalent to the solids that ATK can deliver.
For a national system, buying American is the best way to get funding, unless there is a really good alternative you want to use. For the Atlas V, there really is no good American alternative. Same for the RD-181 for Antares.
So it is really pragmatism. The Russians have some great engines using LOX/Kerosene and the Americans do not have good alternatives.