Why are manhole covers always placed in the middle of the streets?

1) What do you mean here "the middle of the street"?
Do you mean really the middle, at an equal distance from the sidewalks, at about the place where the middle line of the street is drawn to separate the part where the traffic goes in two different directions in the normal case?
This would not be my observation, where I live (Munich) and also in many places that I remember (France for instance).
I rather observed that the manholes were on the sides of the street.

Moreover, it can also happen that the manholes are on the sidewalks.

2) "Manholes are generally found in urban areas, in streets and occasionally under sidewalks."
Source and further information:
http://en.wikipedia.org/wiki/Manholes

Further information:
http://en.wikipedia.org/wiki/Manhole_cover

3) Here an example with manholes in the centre of the street:
"Almost every block contains one or more manholes placed in the centre of the street."
http://query.nytimes.com/mem/archive-free/pdf?_r=1&res=9F02E4DC163FE633A25757C0A9679C946296D6CF&oref=slogin

4) "Plans shall identify those areas in which conduit/manholes will be placed in decorative sidewalks"
Source:
http://www.cityofseattle.net/transportation/docs/Standardsforplansforutilitypermits.doc

5) "Innovative Aspects: "
"It creates the premises for number of reduction of manholes placed on roadbed by moving it to borders."
Source:
http://www.tekes.fi/partner/fin/search/nayta_haku.asp?hakuid=32556