Why did the "institutional church" ever adopt such holidays and symbols of pagan origin?

Let us allow them, the Roman Catholic Council of Nicea, to answer that... my comments in []

And truly, in the first place, it seems to everyone [RCC] a most unworthy thing that we should follow the customs of the Jews [TORAH] in the celebration of this most holy solemnity, who, polluted wretches! Having stained their hands with a nefarious crime [crucifixion], are justly blinded in their minds [murderers]. It is fit, therefore, that rejecting the practice of this people [TORAH & YHWH's Festivals], we should perpetuate to all future ages the celebration of this rite, in a more legitimate order [man-made decree & traditions], which we have kept from the first day of our “Lord’s” passion [the FIRST Believer's were ALL JEWS for over 14 years] even to the present times. Let us then have nothing in common [a NEW  religion] with the most hostile rabble of the Jews. (Council of Nicea, pg. 52.)

This statement of the Council of Nicea is the direct result of the REJECTION of Torah and the mannerisms and customs of YHWH that began with Constantine's brutal attempt to take over the "church" for political gain. Although many Jewish and Gentile believer's in the Messiah accepted Constantine's demands in fear of their lives and for the safety of their people, the persecuted REMNANT,  true to the Torah-based faith in the Messiah Yahushua (Jesus), scattered and continued to keep the "faith delivered once to the saints." The symbolism above is but a few of the "FRUIT" by which you are to JUDGE a TREE to see if it is "GOOD." Although it looks beautiful and rich with flavor, inside it is rotten and putrid, for it has long ago been DIVORCED from the will and methods of worship YHWH and the Messiah require.

Choose this day whom you will serve, YHWH or man's religion, for you are no longer in ignorance!

Want more info? Check out "The Two Babylons"