Why would a Su-25, which has more power and is much lighter than an A-10, only fly half as high?
Just look at the wing loading: The Su-25 has between 585 kg/m² (regular Su-25) and 691 kg/m² (Su-25TK) when fully loaded, whereas the A-10 has a wing loading of 316 kg/m². This alone should explain why the Su-25 cannot climb as high.

Even when flying at minimum mass, the Su-25 has still 309 kg/m² (345 kg/m² for the Su-25TK), and the A-10 is almost a glider by comparison, having just 208 kg/m² at minimum mass.

Since both are subsonic designs, they have a strict limit on maximum speed. With that, wing loading and the airfoil determine their ceiling if thrust is sufficient.

Another limiting factor is the cockpit: Since the Su-25 was designed for ground attack, it has an unpressurized cockpit. This detail limits the certified envelope, even though a lightly loaded Su-25 can climb higher than 7000 m.