Money, profits, the market.
They make what sells. Traditional money making molds, Spitfires, Mustangs, Shermans, Tigers are going to be produced by the mainstream companies.
New companies trying to get a foothold in the market often go for the popular best sellers if the market is ripe for the update to that subject. If there is the "ultimate" kit of that subject already on the market (Dragon's M1A1AIM & M1A2SEP jump to mind), then they may go in a different direction.
Sometimes the new company goes for a niche that has potential growth and is untapped. Dragon's first models were modern Soviet armor, a field ignored by the major manufacturers at the time. Their big money makers were the T-72, T-80, BMP-1, BMP-2, BTR-70 and BRDM. Cutting edge stuff considering the Wall was still up and references were scarce. Those kits sold well and made a lot of money for Dragon. Over twenty years later, only the Tamiya T-72 has has bumped any of those ex-Soviet vehicles from the shelves.
In today's economy, new kits require taking a chance. Fortunately for armor modelers, the mainly ground war seen in OIF/OEF/OND gives a new area of MRAPS, Strykers and constantly changing field modifications. New companies are taking chances in these areas and hitting homers.
Other companies are content to sit back and re-pop tried and true kits. Maybe tossing in a few new bits to make another version (Tamiya's M1A2, M113A2 and M2A2 ODS jump to mind here).
It takes thousands of dollars to make new kits. The companies have to be sure the market is there before taking the chance. Two companies took a chance on one of Tamiya's most popular sellers, the M41 Walker Bulldog.
It is a kit long past its prime, but was still a big seller and found everywhere. The volume sales of this old starter kit probably made them more money than their outstanding Tiger series from the nineties.
AFV Club and Skybow both took a chance at making the next cutting edge model of this popular seller. They hit the market simultaneously and neither one sold very well. It probably put Skybow out of business (it was their first tank). AFV Club had enough to absorb the losses. Both kits are outstanding models and died on the shelves.
What they didn't realize was that the Tamiya kit wasn't popular because of the subject. The kit was popular because it was cheap, easy to build, readily available and looked like a generic US Army tank.