There will never be a lot of Kubota toys unless Kubota themselves goes to companies that produce small runs of limited sale models. CNH and Deere have a ton of models because RC2 (the company that owns Ertl), Siku, Universal Hobbies and Scale Models all go to them looking to make models of their tractors. Agco doesn't have as large a toy market as Deere and CNH but has Spec Cast that does a ton of models for them.
The problem with Kubota as well as most smaller tractor companies (yes, Kubota is considered a smaller tractor company) is that there is very little market for the toy manufacturers to sell their tractors to. The reason they are so eager to go with Deere and CNH is because most of the dealers as well as toy dealers and farm stores will carry these models. They can make tons of them which keeps the price down. Kubota has a small, select following that limits the marketing potential for models so only very large toy dealers would be willing to carry their models as well as select Kubota dealers. Since the models are small runs they cost more to produce which makes them cost more and as a result means less people will buy them (it is a strange circle that only Kubota can really control). Since not a lot of people are buying or even looking for Kubota models there is no incentive for toy manufacturers to want to produce every model of Kubota tractor ever made like they do with Deere and CNH. Deere spends a ton of money marketing their models and brand which is why they have so many models, CNH does a good job also but is still way behind what Deere does. Agco is third by a considerable margin.
One thing that Kubota owners can do is contact small companies like Scale Models, Universal Hobbies, Die Cast Promotions and Spec Cast and tell them what you want. Also, tell them what other models of tractors they can get by using the same tooling (like the John Deere line, they can get a bunch of models by basically changing the decal or roof and hood panel). The more models they can get out of one set of tooling the better as then they can release "new" models every so often by just making a minor change.
I will as Bill Walters (he is the one in charge of what farm models get produced by RC2) what demands he gets for Kubota models or if Kubota has ever contacted them to have models made and let you guys know.