Gaining an additional 5% is a huge thing for any market share.
It depends on how profitable that extra 5% is. If it costs 1Euro to get it and makes .1Euro profit, if there is another opportunity to invest that 1Euro that makes more than .1Euro profit you should forgo the added market share and invest in the other opportunity. You invest where you get the best return on your investment.
In addition, MS wants to sell you other solutions and making alternatives to those solutions readily available runs counter to that strategy and risks eating into the desired solution's sales; thus no reason to go after that market and good ones not to.