Syncretism in the ancient world was something quite different from modern syncretism.
First, and probably most important, is the difference between the way moderns frequently view religion and the way in which the Romans of antiquity did. For moderns religion is more about themselves than about the Gods: it is a route to self-realisation, self-fulfillment, a way to make one happy about oneself. The focus is primarily on the self. If religion is primarily about the self, then selecting what pleases oneself from a variety of religions with only the principle of pleasing oneself as the deciding criterion. In antiquity the primary focus was on the Di Immortales, on following the tradition cultus handed down from time immemorial to ensure maintenance of the pax Deorum. The emphasis was on pleasing the Gods and keeping entact humanity's contracts with them. To be sure, men supplicated the Gods for what they needed and desired, but man's ability to do so was grounded in making sure that the Gods looked favourably upon them by adhering precisely to age-old rituals and formulae. Do ut dare was predicated precisely on recognition of an asymmetry which required that the Gods be placated before they were supplicated. This is very different from the modern view.
Second, Roman syncretism was based on theological presuppositions different from modern syncretism. Keith Hopkins was right when he characterised antiquity as "a world full of Gods." A polytheism which saw divine entities everywhere was the norm. Furthermore, many of these deities were associated with place. In such an environment it was only natural that parallels between deities' functions and characteristics would be noted and the idea that, for example, the Roman Minerva and the Britannic Sulis were the same divine being, Minerva Sulis, arose. This kind of natural syncretism was commonplace in the Roman world, and appears to have been encouraged in the process of Romanisation. There was also the notion that foreign Gods had power: there is evidence for ritual condemnation of the Gods of foriegn peoples at war with the Romans, aimed at the Di Immortales subjugating the foreign Gods just as Roman armies subjugated foreign peoples. It is unsurprising, then, that once a people was conquered and the process of Romanisation begun the assimilation of their foreign Gods to the Roman pantheon would occur. This did not happen in every case, but, again, it was the norm for conquered peoples.
Third, the introduction of foreign cults into the religio publica was primarily a function of prodigy. A grave crisis or miraculous portent signaled the acceptability of a foreign cult's alliance with Rome and her Gods -- Magna Deorum Mater during the Punic Wars and Isis in the reign of Gaius Caligula are two salient examples. Worship of the foreign was always regarded as aberrant by the Romans unless the foreign cult was part of the officially sanctioned sacra peregrina or a result of Romanising assimilation.
Fourth, the first great monotheism, Judaism, provided mightily impervious to such syncretism in the Roman world precisely because it rejected polytheism absolutely. When there was a Sanhedrin, a Jew who embraced avodah zarah (and both the Hellenic cults and the Religio Romana were archetypes of avodah zarah, as even the most cursory examination of the Talmud reveals) was subject to capital punishment. Judaism's definition of the divine excluded the polytheism which was at the Religio Romana's base. Likewise, the second great monotheism rejected Roman syncretism. As Cato has pointed out, Christianity denies that other religions, the Religio Romana included, are paths to the divine. If Christianity had been acceptant of polytheism, most Romans would have regarded it as less of a threat, and there would have been rather fewer Christian martyrs. The chance for such an accommodation died with Arianism. The syncretism of Christianity was not the positive syncretism of the Religio Romana, but rather a negative syncretism: the church was prepared to coopt rituals and holidays of the polytheistic world in order to make its message more palatable to a polytheist audience, but it always changed these borrowings in such a way as to focus on Christianity's exclusive claim to the divine.
Fifth, ritual magic was abhorred by the Romans, most especially in the Republican period, because it was associated with invocation of malevolent spirits to work mischief or greater evils; veneficium was the generic term for such magic, and came to be associated with murder by poisoning. Ritual magic was the province of foreigners; some Romans might stoop to purchasing their services, but the social stigma if discovered was enormous.