Why would you say that? The statistics I've seen say Linux is on around 2.5% of desktop computers, with Windows on over 90%. So I don't see any evidence that desktop software vendors are being irrational by focusing on the Windows market, or that they would have any bias against developing for Linux if they thought there was money to be made there. I do agree that companies could help increase Linux's usage if they wanted by making lots of apps for Linux, which would make using Linux more attractive for ordinary users. However only big companies with deep pockets can really afford long-term strategies like that, since even if more companies port their software to Linux, it would take many years for this to have the effect of increasing Linux's market share significantly. It's just not reasonable to expect small software makers to be able to do that.

And in any case what would be their motivation to do that? Unless a company is in direct competition with Microsoft, or they have some personal mission to unseat Microsoft, there's really no incentive for a company to go out of its way to try to increase Linux usage.