Why did Joseph Smith return the golden plates to the angel to be buried?

First, to clarify, the ancient record was recorded on plates, bound by rings, not on tablets. I've never seen official or first-person historical sources refer to them as tablets and have only seen that term used in sources which have either strong or subtle sentiments against the Mormon church, or a political agenda. From every reliable source I have ever seen, plates is the proper term. (The distinction is important because the Book of Mormon was actually a book, not a slate. Tablets are not plates. Plates are thin, light, and can be bound together. Tablets are like what the 10 Commandments or Hammurabi's Code were written on... the plates were bound into a codex.)

Joseph was instructed from the beginning to the end that the plates held a very singular purpose: to bring to light great truths which were otherwise lost. From Joseph Smith—History, Joseph Smith was warned not to use the plates for gain.

46 By this time, so deep were the impressions made on my mind, that sleep had fled from my eyes, and I lay overwhelmed in astonishment at what I had both seen and heard. But what was my surprise when again I beheld the same messenger at my bedside, and heard him rehearse or repeat over again to me the same things as before; and added a caution to me, telling me that Satan would try to tempt me (in consequence of the indigent circumstances of my father’s family), to get the plates for the purpose of getting rich. This he forbade me, saying that I must have no other object in view in getting the plates but to glorify God, and must not be influenced by any other motive than that of building his kingdom; otherwise I could not get them.

As for the follow-up question, pondering on "the street value of the gold" actually caused delay in the Restoration process and Joseph was punished for entertaining the thought (see heading: "Joseph Smith—History 1:50–53. Joseph’s First Visit to the Hill Cumorah):

“As Joseph approached the Hill Cumorah, he had thoughts about the poverty of his family and the possibility that the plates or the popularity of the translation would produce enough wealth to ‘raise him above a level with the common earthly fortunes of his fellow men, and relieve his family from want’ [Oliver Cowdery, in Messenger and Advocate, July 1835, 157]. When he reached down for the plates he received a shock and was thus prevented from taking them out of the box. Twice more he tried and was thrown back. In frustration he cried out, ‘Why can I not obtain this book?’ Moroni appeared and told him it was because he had not kept the commandments but had yielded to the temptations of Satan to obtain the plates for riches instead of having his eye single to the glory of God as he had been commanded [Cowdery, in Messenger and Advocate, Oct. 1835, 198].

“Repentant, Joseph humbly sought the Lord in prayer and was filled with the Spirit. A vision was opened to him, and the ‘glory of the Lord shone round about and rested upon him. … He beheld the prince of darkness. … The heavenly messenger [Moroni] said, “All this is shown, the good and the evil, the holy and impure, the glory of God and the power of darkness, that you may know hereafter the two powers and never be influenced or overcome by that wicked one.” … You now see why you could not obtain this record; that the commandment was strict, and that if ever these sacred things are obtained they must be by prayer and faithfulness in obeying the Lord. They are not deposited here for the sake of accumulating gain and wealth for the glory of this world: they were sealed by the prayer of faith, and because of the knowledge which they contain they are of no worth among the children of men, only for their knowledge’ [Cowdery, in Messenger and Advocate, Oct. 1835, 198]. Moroni concluded by warning Joseph that he would not be allowed to obtain the plates ‘until he had learned to keep the commandments of God—not only till he was willing but able to do it’ [in Lucy Mack Smith, History of Joseph Smith, 81; italics added]” (Church History in the Fulness of Times, 40–41)

And the fate of the plates?

It has not been mentioned that the plates were then buried. Rather:

When, according to arrangements, the messenger called for them, I delivered them up to him; and he has them in his charge until this day, being the second day of May, one thousand eight hundred and thirty-eight.

So to answer the question... why did he return the plates? Because he was commanded to do so. Why? We're not entirely sure... the Lord does mention that He would try the faith of His people:

6 And now there cannot be written in this book even a hundredth part of the things which Jesus did truly teach unto the people;

7 But behold the plates of Nephi do contain the more part of the things which he taught the people.

8 And these things have I written, which are a lesser part of the things which he taught the people; and I have written them to the intent that they may be brought again unto this people, from the Gentiles, according to the words which Jesus hath spoken.

9 And when they shall have received this, which is expedient that they should have first, to try their faith, and if it shall so be that they shall believe these things then shall the greater things be made manifest unto them.

10 And if it so be that they will not believe these things, then shall the greater things be withheld from them, unto their condemnation.

11 Behold, I was about to write them, all which were engraven upon the plates of Nephi, but the Lord forbade it, saying: I will try the faith of my people.

And to answer the next question from your comment, both the Large and Small Plates (which were concatenated by Mormon) were full, so Joseph Smith could have written no more on them:

Omni 1 (small plates)

30 And I, Amaleki, had a brother, who also went with them; and I have not since known concerning them. And I am about to lie down in my grave; and these plates are full. And I make an end of my speaking.

Mormon 8 (large plates)

5 Behold, my father hath made this record, and he hath written the intent thereof. And behold, I would write it also if I had room upon the plates, but I have not; and ore I have none, for I am alone. My father hath been slain in battle, and all my kinsfolk, and I have not friends nor whither to go; and how long the Lord will suffer that I may live I know not.