According to mainline Protestant theology, why is Pentecost on Sunday and why did it lose its significance? Christian Pentecost

It is well known that the early Christians were Jews who accepted Jesus, who was also a Jew, as the messiah. These Christian Jews also observed Shavuot, a Jewish holiday but later changed the name to Pentecost (its Latin name). The New Testament Book of Acts 2 explains that the Holy Spirit descended upon the eleven apostles (since the twelfth, Judas', passing). Thus Pentecost is “the birthday of the church.” Interestingly enough, this is similar to the modern Jewish concept of Shavuot which commemorates the revelation of the Torah.[1]

Of course, the original meaning of Shavuot was not to celebrate the giving of the Torah at Sinai since the Torah never selected a date for the Sinai revelation. Nevertheless, this was the meaning or how Shavuot was practiced (and still is) during Second Temple times (ie the times of Jesus). 

Leaving aside the fact that Shabbat was originally on Sunday and that the rabbis later changed it to Saturday and Christians back to Sunday, why don't Christians (who consider themselves to be spiritually chosen, in a sense, nevertheless, keep Pentecost on Saturday, “Sabbath” since this was the date chosen by the rabbis? And why don't Christian also celebrate it as the date of the giving of the Torah?

[1] Christians also have a tradition that the Apostles were dressed in white, thus calling Pentecost Whitsunday (white Sunday).
The count to Pentecost involves a start and an end requirement in order to reach 7 weeks of 7 plus 1 day to equal 50 days.  There's only one way to achieve that; start on a Sunday and end on a Sunday with 7 weeks of 7 between those two days.
THE START

And he shall wave the sheaf before the LORD, to be accepted for you: on the morrow after the sabbath the priest shall wave it. Lev. 23:11

THE COUNT

And ye shall count unto you from the morrow after the sabbath, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete: Lev. 23:15

THE END

Even unto the morrow after the seventh sabbath shall ye number fifty days; and ye shall offer a new meat offering unto the LORD. Lev. 23:16

If one starts on any other day, one cannot achieve the start, count, and end requirements.  The start is always after the weekly Sabbath, the count always requires 7 sabbaths, and then the day after the 7th sabbath or 49th day, which is always a Sunday.
So, this is the reason Pentecost always falls on the first day (aka Sunday).
SABBATH/7th DAY
Sabbath (Shabbat) is on the 7th day (aka Saturday) with day 1 (aka Sunday) per creation week (Gen. 2:2-3).
IMPORTANCE
A number of Christian denominations continue to honor Pentecost, including Catholic, Orthodox, Pentecostal, Lutheran, Evangelical, and many more.
PS To be sure, there are some groups who believe that Pentecost may fall on other days of the week besides Sunday, but anyone may do the count and find it must start and end on a Sunday with 7 weeks of Sabbaths plus 1 day to get 50 days.
In addition, THE END requirement details that the 50th day is the day after the Sabbath.  That must be the referring to the weekly Sabbath and not to a feast Sabbath because there are no feast Sabbaths at that time, let alone on the 49th day.  Thus that 49th day must needs be another weekly Sabbath.
OBSERVANCE
On the question about why Christians don't celebrate the giving of Torah on Pentecost, but rather focus on the giving of the Spirit on Pentecost, the answer is two fold about the old and the new.
The Law was given to Israel and it would be their righteousness to follow it.

And it [the testimonies, statutes, and judgments v20] shall be our righteousness, if we observe to do all these commandments before the LORD our God, as he hath commanded us. Deut. 6:25

Romans and Galatians and Hebrews contrast the Law and the Spirit in numerous ways.

That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit. Rom. 8:4
But if ye be led of the Spirit, ye are not under the law.  Gal. 5:18

So, while Christians may understand that Torah was given at Mt. Sinai on Pentecost, we rather observe the giving of the Spirit to all who believe.

And when the day of Pentecost was fully come, they were all with one accord in one place.... And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance. Acts 2:1,4
Then Peter opened his mouth, and said, Of a truth I perceive that God is no respecter of persons:... While Peter yet spake these words, the Holy Ghost fell on all them which heard the word. Acts 10:34, 44