Ganga is a holy river. All the cities which are on the banks of the river Ganga should be sacred but this is not the case. For example Varanasi is a sacred city but Kanpur is not, even though both are on the banks of Ganga. Ideally all the Ganga cities should be sacred but it is not so. Why ?
Puranas say Ganga is indeed holy and bestows her grace on those who have faith in her. For example, Kashi khanda of Skanda Purana says Ganga contributes to welfare depending on level of devotion. It even says one needs some great vasanas to get born/stay of banks of Ganga.
Iswara (Shiva) says the following to Lord Vishnu in Chapter 27 of Kashi Khanda of Skanda Purana:
A person who bathes in Gangä regularly is superior to everyone in every Varna and stage of life, to everyone among vedic scholars and to everyone who has mastered the sacred scriptures and the topics discussed therein.
Even a person defiled by many kinds of evils originating from mind, speech and body, becomes.sanctified by seeing Gangä. There is no doubt in this.
In Krityuga there are holy Tirthas everywhere; in Tretayuga Puskara is the greatest of all Tirthas; in Dväparayuga, Kurukshetra (is the best) and in Kaliyuga only Gangä (is the holiest).
O Hari, one shall have (the opportunity to) live on the banks of Gangä if there is the Väsanä (impression left after repeated practice) of the previous births, with my great favour.
Obviously all these will show results only when people have faith and devotion towards Ganga.
- O Visnu, Gangä contributes to the welfare of all the worlds. She alone bestows benefits here as well as hereafter on all living beings in accordance with their devotional fervor.
- O Govinda, Gangä becomes pleased with the devotion of one who understands the greatness of Gangä even if he were to be an undeserving person, even if he stays far off.
- Faith is the greatest and subdest Dharma (virtue). Faith is the greatest penance and knowledge; faith is heaven and salvaüon. She becomes pleased with (one who has) faith (in her).
And Shiva says explicitly all spots are auspicious in same chapter.
- On the banks of the glorious Gangă all times are auspicious, all the spots are auspicious and holy, all persons are fit for the receipt of religious gifts.
So, it says one who takes bath at her banks (be it anywhere), Ganga bestows welfare. But there are some Kshetras where some legends occurred such as Kashi (Varanasi), Prayaga, Mayapuri (Haridwar) and so on and sacredness varies depending on the place.
For example, Kashi being city that was created by Nirguna SadaShiva and Ganga flowing northwards is considered very sacred. You can read complete Kashi Khanda to know sacredness of Kashi.
Chapter 32, Kashi Khanda (part 1) of Skanda Purana says:
- Gangä flowing towards the North at Käsi and my Linga named Visvesvara in Käsi, are the two which bestow salvation on men and they are to be attained in Kaliyuga through the power of Dana.
Similarly, there are legends associated with other holy Kshetras such as Prayaga, Mayapuri (Haridwar) that are on banks of Ganga but Ganga is holy at all places and some places are more sacred when compared to other places.
Your main enquiry is why the holiness og Ganga to all the cities on its bank ? Ideally all the Ganga cities should be sacred but it is not so. Why?
It is not so , all the cities at the bank of holy Ganga river are holy places and are called as Siddha-places.
The very specific answer to your question is provided in Mahabharata- Vana Parva- Tirth Yatra Parva- Chapter 85 - Verse 97 - Which is telling us mahtmaya of Ganga river.
यत्र गंगा महाराज स देशस्तत तपोवनम |
सिद्धक्षेत्रं च ताज्ञ्नेयं गंगातीरसमाश्रीतम ||97 ||
P. 195 - There is no tirtha that is like unto the Ganga, there is no god like unto Kesava, and there is none superior to Brahmanas,--this hath been said even by the Grandsire. O great king, the region through which the Ganga flows should be regarded as a sacred asylum, and a spot of land that is on the Ganga's banks, should be regarded as one favourable to the attainment of ascetic success.
So its very well clear that all the cities at the bank of the Ganga river are sacred and are considered as holy places and best for attainment of ascetic success.