Why can't be produce 15kv, 70 kv??

They produce various amounts. Are you thinking 11kv is the limit?

Generally their generating ststion produces will be seen in 11kv, I need reason??

Generators are designed to produce standard voltages like 400,480,6600,11000 etc depending on the rated power as at higher voltages current will be less.

From the past 11kv is very popular unit for transmission as every products are avelable in 500grams,1kg peckets same as for power.