Mostly because it is a well characterized wild type strain of E.coli. However you can safely use a variety of other strains if you prefer, there is no reason you need to use MG1655.

Because it is a strain widely characterized and used in the laboratory. What I recommend is that you review the genotype in detail in case you are making modifications in the genomes that could generate incompatibility of the strains and check if the strain has an active, modified or inactive Restriction-Modification System, since that will help you to increase the conjugation rate.