The simplest answer is that nothing better has come along. You have to remember, the tank was only invented in the mid 1910's. A technology in its infancy will often go through rapid change as new advances build on each other, and repeated use of the technology shows the best way to use it and improve it. Then, as the technology matures and is perfected, the rate of advancement slows. There's just not as much to perfect.

This isn't unique to tanks, either. Look at military aircraft. The dominant fighters in the US inventory are the F-15, 16, and 18. The -15 and -16 entered service in the mid 70's, the -18 in the mid 80s. And all of those platforms are still going to be around for at least another 20 years. Small arms, the AK-47 was first produced in the '40s. The M-16 was designed in the '50s. Both of those weapons are still carried in some variant by many armies around the world.

Until a major change in how we do battle comes about, we just have the best tools for warfare already in hand.