Analogs of thymidine such as BrdU or Tritated Thymidine are used to measure DNA synthesis because they can be labeled and they readily integrate into DNA, replacing thymidine. Analogs of other bases are never used for this measurement and many of them are chain terminators. What is special about thymidine that allows DNA replication to go forth when modified analogs are put into solution?
It's a very interesting question, something I never really wondered about until somebody points it out, so I gave it a thought.
Thymine has a methyl group (CH3). When pairing with adenine, this methyl group points "away" and is not involved in any bonding. No other base has such a modal group that is not involved in the bond, probably explaining why it can be more easily replaced by a "tag" - indeed this is the group that is replaced by bromine in BrdU and ethynyl in Edu.
The reason the methyl group is not involved in the bond is because it's also pretty much just another "tag". It makes the difference between thymine and uracil: thymine is essential 5-methyluracil. In DNA, thymine is used instead of uracil because cytosine can spontaneously de-aminate to uracil. In short-lived RNA, this matters little, but in DNA, there are repair mechanisms in place. Having methyluracil/thymine in DNA in stead of uracil allows these mechanism to distinguish between "degraded Cs" and "true U/Ts".
So the "tag"-group does not contribute to base-pairing and can be modified.
In contrast, other modifications of nucleoside/nucleotide can result in
(A) chain termination due to a lack of 3-OH group on the sugar (e.g. dideoxynucleotides in Sanger sequencing)
(B) inhibition of base pairing, resulting in a destabilized DNA product (as is the case with some antiviral drugs for example)
I am sure there are exceptions and intricacies that I am not aware of, but I hope this helps to answer your question!
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