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The 11th ed. of the Encyclopaedia Britannica (published in 1911) provides a specific answer as to when. The why can perhaps be inferred from its somewhat anti-Turkish language (tortuous, mean) and the historical events around independence in 1879.
Since 1880 the city has been almost entirely renovated in the "European" style; the narrow tortuous lanes and mean houses of the Turkish epoch have almost disappeared, and a new town with straight parallel streets has been constructed in the eastern suburb.
As for by whom, e.g. various Wikipedia articles provide names of architects who were invited to work in Bulgaria (e.g. Friedrich GrΓΌnanger).
While the extent of "renovation" may be larger than elsewhere because of politics of the time, other European capitals also saw considerable renovations in the 19th century, e.g. during Georges-Eugène Hausmann's modernization program in Paris (in the 1860s) or during expansion of the city center in Vienna (after razing of the city walls after 1857). This was also a time when railroads significantly expanded, and this also must have had major architectural effects on many cities.