A fission bomb must start with a subcritical assembly and detonates by making it become supercritical. If you want a large amount of fission fuel, making a shape that's not already supercritical is a significant design constraint.

Economics is also a factor. A pure fission or boosted fission weapon gets most of its yield from the fission of either highly-enriched uranium or weapons-grade plutonium. Those are expensive.

(Boosted fission = you put a little bit of fusion fuel in the middle. It doesn't make much yield directly but it makes lots of neutrons to cause more fission in the plutonium. Without boosting only a small percentage of the fuel actually fissions.)

A thermonuclear weapon, AKA hydrogen bomb, uses a (nowadays always boosted) fission primary to cause a larger fusion reaction. Typically, but not always, the fusion then creates additional fission in a natural or depleted uranium tamper, much cheaper per kg than highly-enriched uranium. The yield is predominantly from the fusion and the tamper fission, and a relatively small amount of expensive material is needed for the primary.

Almost all current nuclear weapons are thermonuclear. It's considered to be the best design even for relatively low-yields of a few tens of kilotons. I think North Korea and possibly Pakistan are the only states using pure or boosted fission weapons.