The Church recognizes and understands that many people that investigate the Church have already been baptized in other churches.  
 
While it is a great thing that you have shown your devoting an love for Christ by taking that step, the Church of Jesus Christ of Latter-day Saints requires all converts (myself included) to be baptized by someone holding the proper priesthood authority of God.  
 
This Priesthood authority to bind on Earth and in heaven is only found in the one true Church of Jesus Christ, which is the Church of Jesus Christ of Latter-day Saints.  
 
While baptisms by other churchs are important for growth and progression there clergy do not hold the priesthood authority of God, so there ordinances do not hold the power of God behind them only the power of man.  
 
A good analogy is if you are speeding on the Highway a construction worker, a milkman, anyone can see that you are breaking the law, but only a police officer has the authority to give you a speeding ticket.  
 
The Construction worker, the milkman, or anyone else most definitely can also write up a ticket saying you were speeding but it has no lawful authority behind it, the judge would toss that ticket out in court because it wasn't done by the proper authority.  
 
The same goes for baptisms and all the ordinances they have to be done by the proper priesthood authority and that Authority rests in the Melchizedek and Aaronic Priesthoods in the Church of Jesus Christ of Latter-day Saints.  
 
Also like to note that no other Church recognizes an LDS baptism as authorized eitehr.