GSM: Why is the channel spacing in GSM 900 just 200KHz?

I've came across this question so many times in the period of telecommunication learning. Every time I asked the question, there was no one to properly answer the question, may be it was because I couldn't phrase the question in easily understandable way. Eventually I came across a few answers, here is the most convincing one from quora..

Why the channel spacing in GSM 900 is just 200KHz ?

It was a trade-off: wider channels = more bandwidth and more users.


Narrower channels are easier to implement (the equalizer) *

Other, very similar TDMA/FDD chose different channel sizes:
US D-AMPS / IS136 used 30KHZ (but only had three users instead of 8 or 16)
Japan's PDC used 25KHz, again for 3 users

GSM beat both of those. I don't know that proves 200KHz was "better"

There is no unique "right answer"

If you do an archive trawl through 3GPP standards from early 90s you'll probably find the analysis (I wish you luck...!)

* Remember these decisions were taken 20 years ago: the technology at the time is relevant, not what you can do now.

NOTE
It uses 25MHz each in 890-915 MHz and 935-960 MHz ranges with 200khz channel spacing 
That is not completely correct: GSM uses many more ranges than that e around 850 and 1800.