Your question as stated, is rather confusing. I assume that what you mean is why thymine is replaced by uracil in RNA? If not, you will have to elaborate!

The short answer is that I do not know; that is a rather specialist question. The following remarks are the purest guesswork and I hope you have enough salt to take them with.

The first thing I observe is that thymine is essentially methylated uracil. Now, putting it vaguely and generally, the methyl group in nucleic acids is like a cork on the sharp end of a practice weapon; its function is largely to prevent, or at least modulate, unwanted effects. This can be to disable certain functions, or it can be to make them more selective.

Consider by way of analogy, the design of a key. It requires two things; it must have gaps where lack of a gap would prevent its entry into the lock in functional positions, and it must have substance where a material projection is needed to push whatever must be pushed to work the lock. Thymine and uracil form very similar hydrogen bonds, but the methyl group on thymine gets in the way when it otherwise would pair cheerfully, but indiscriminately, with other bases. In practice its methyl group makes it bashful in the presence of anything but adenine. Such selectivity is vital in DNA, but RNA is a very different matter.

Uracil is a far more promiscuous base, and will pair with practically anything. This has little to do with the function of RNA, which occurs largely in unpaired chains, but it would be fatal to the transcription function of DNA.

Conversely, RNA has little to do with transcription in that sense, so the presence of uracil hardly matters, but on the other hand its single-stranded functions often involve the formation of (often apparently tangled) tertiary structures resembling those of proteins, and as in the case of proteins, hydrogen bond cross-linking can play many crucial and versatile parts in determining and stabilising such structures. No doubt uracil is valuable in many such positions, where its base-pairing functions would be irrelevant. Sorry to be so vague, but...

All that of course, subject to anyone knowing better...

The current thinking is that RNA came first: so we need to reverse the question. Why does DNA contain thymine rather than uracil? One suggestion is that cytosine can undergo deamination, and become uracil. In an RNA molecule there is no obvious clue that this has happened: both are normal bases. In DNA, by replacing uracil with thymine a deamination becomes obvious, since there should be no uracil present and its presence means that it needs to be repaired.

BTW the switch from ribose to deoxyribose probably relates to the fact that double helices with RNA (DNA:RNA or RNA:RNA) do not form as tight a bond as the DNA:DNA structure (generally forming the looser A structure, rather than the B structure typically seen in DNA). This is useful since it means that the RNA made in transcription is readily displaced from the DNA template by the other strand of DNA. If both boded equally well, then the RNA would tend to remain attached, rather that being released.

Mike, your remark on the bonding between RNA cains as compared to DNA chains is fair, but your remarks on conversions between cytosine/uracil/thymine make no sense to me at all, least of all functionally in NA chains. Would you care to rephrase or elaborate on that point please?

I think that part of this event about thymine and uracil is related to Evolutionary road of genetic material. We know that that RNA came first and there was an "RNA world" before DNA evolved. After DNA evolved, thymine may have proved a preferable material for storing genetic information because of its much greater stability; RNA breaks down relatively quickly, but DNA is stabilized by its double-stranded form.I addition, Thymine base pairs only with Adenine. Although Uracil shows higher affinity for Adenine, it can also bind to cytosine, guanine and another uracil so chances of errors during replication is possible if uracils were to be present in DNA.Also, the methyl group in thymine has a protective role for DNA.