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There is a logic in that.
Enc. Brit. writes this about Bakhmut:
> The town originated in the 17th century as a fort protecting the Russian frontiers against the Crimean Tatars. Peter I (the Great) established a salt industry there in 1701, but seven years later the fort was destroyed in the Bulavin revolt. It officially became a town in 1783. Salt operations were revived in the 19th century and gave rise to a chemical industry based similarly on local supplies of limestone and coke. There are also metalworking and light industries.
https://www.britannica.com/place/Bakhmut
I got the feeling that fort was more for garrisoning troops so they can strike out on the Crimean Tatars who weren't much into siegecraft at that point.
Bakhmut lays at the Bakhmutka river which joins into the Donetsk. This gave an importance to the town, since waterways were used similarly to railway lines before railroad existed. And depending the river can be used as a defensive line. However the town is down in the valley, which isn't really a good lay for defense.
I think we should seek other explanations why it holds so long, and the explanation is in the nature of this war. It's like the trench warfare of WWI and the urban warfare of WWII, with the technical possibilities of our day and age (it's use and the lack of use).
The mines around that region, and the tunnel system that might joins those together is also a suspect on my behalf. But those tunnels (if they exist) are the leftovers of the Soviet era.