LTO Tapes have typically been doubling in capacity right from LTO-1 generation , at least upto now. This changed with LTO-6, where the capacity hasn't exactly doubled. The reason lies in the technological limitation Metal particles have.
LTO tapes have been using Metal Particle technology from Gen1. With every generation capacities were doubling, and to do that you need to increase the linear density of the tape so you can cram more data in the same space ( or you can increase the tape length too , but that has other problems). Problem starts when you can't increase the linear density of the tape.
LTO-5 at 1.5TB started to approach the maximum linear density that can be achieved using metal particles. In fact LTO-6 has the same linear density as LTO-5 . It was all well till LTO-6, but for future generations, the manufacturers had to look for something that could give them better linear density , and they came up with Berium Ferrite (BaFe).
BaFe is a newer technology that can offer much greater linear density compared to MP. LTO-6 has both MP and BaFe variants ( some vendors like HP are offering both in their product line). But the future generations would all use BaFe , unless MP linear density can be increased.