The word mandarin is the English corrupted pronunciation of man da ren (滿大人)﹐which literally means Manchu Big Man (big shot, an important person ). The spoken language used by the officials of Qing dynasty of China was the spoken language by the educated residents of Beijing based on the dialect of Beijing. The uneducated spoken language of the people of Beijing is difficult to understand.  Today the common language is called pu tong hua (general spoken language).  Today, mandarin refers to the general spoken language of China, not an ethnic group.  The province of Canton (Guangdong 廣東）has many dialects. It is difficult to imagine the Qing dynasty had the idea of choosing Cantonese and Mandarin. When they settled in Beijing, it is reasonable they were exposed to the local dialect of Beijing. The Beijing dialect automatically be the first exposure to the new rulers.
 

Chinese pronunciation has changed over the centuries. During the dynasties, many people migrated to the south to avoid the wars.  It is not easy to move around in the south. In the northern part of China it is a big plain. People move more easily and the language evolves faster. The Chinese language is more preserved in the south than in the north because the south has a lot of mountains, terrains and rivers.

If you look at the Chinese translation of Buddhist sutras from the Tang dynasty, the sound translation does not match today's standard Chinese pronunciation but it matches today's Fujian (Hokkien) pronunciation. In the very older translation the sound is closer to some Cantonese pronunciation.

Last year,  I was on the silk route of China, a county called 庫車 is written officially in pinyin as Kuche. Actually during and before the Han dynasty of China, the word 車 was pronounced as char in those days, almost as the Hakka pronunciation, . Thus the local call it as Kuchar and not Kuche. Wikipedia also uses Kuchar, the local name for the county. The official pinyin is ku che is incorrect. It should written in pinyin as ku cha. You can see how language has evolved here.

Another example is the mantra of Heart Sutra : 揭諦揭諦，波羅揭諦，波羅僧揭諦，菩提薩婆訶。The original sound of Sanskrit is: "Ga te ga te, para ga te, para sam gate, bodhi suaha." Here, the today standard Chinese common language （普通話）sound does not match the Sanskrit sound because during Tang Dynasty the Chinese pronunciation was closer to Fujian (Hokkien) pronunciation. If you read it in Fujian (Hokkien) dialect it is closer to Sanskrit sound.

This famous mantra was transliterated by the monk in "Journey to the West" ( 西遊記），Xuan Chang (Tang San Zhang 唐三藏).during the Tang Dynasty..

Kumarajiva, another monk before the Tang dynasty, translated another longer version of Heart Sutra.  Kumarajiva's name was transliterated as 鳩摩羅什.

Why his name Kumarajiva was it transliterated as 鳩摩羅什? The word 鳩 today is pronounced as jiu (chiew) in standard Chinese, but during that period it was pronounced as koo,  (the first syllable of his name). The pronunciation of 鳩 is like Cantonese for penis, koo koo, a term used by the Cantonese little kids. (The word 鳩, is a kind of bird, but it also means penis in Cantonese slang.). The then spoken Chinese is somewhat close to today Cantonese in some word.

Hokkien language in Taiwan were mainly from Zhangzhou district (漳州) and Quanzhou district (泉州) of Fujian (Hokkien) province. Now the local Taiwan dialect can be a mixture of Zhangzhou dialect (漳州話） and Quanzhou dialect (泉州話）. Some people joke about it as not Zhang not Quan (不漳不泉). It also mixes in some Japanese terms because of Japanese occupation of Taiwan; just like the Hokkien dialect in Singapore, Malaysia and Indonesia mixes with Malay language. We can confuse the people in Fujian and Taiwan when we speak Hokkien dialect too fast when many native words just mixed into the Fujian dialect would come out, like tetapi, mana, aduh, jilakak etc..

Hokkien language in Penang were mainly originated from Zhangzhou 漳州 of Fujian (Hokkien) province. 

If you look at the name of Zhangzhou Association in Penang it is SAM TOH BEAN LOK KEOK
PENANG三都聫絡局梹榔嶼. The word 聯 , we might think it should be transliterated as Lian and we may think the pioneers did not know English. We are wrong. The old Hokkien Zhangzhou 漳州 dialect pronunciation of 聯 actually sounded like the English word bean. Even not too long ago I heard some Hokkien old men pronounced the word Soviet as SO BEAN instead of So Lian.

How the dialect of Hokkien is spoken, depends on where the speakers or their ancestors were from which part of Hokkien province.

We can only say that the ancient Chinese pronunciation is better preserved in the southern provinces of China. The Chinese pronunciation in the northern part of China, because people can easily mingle together, has been changed or evolved.