Here's why:
* Tenacious is an adjective describing someone or something that is determined and persistent, holding onto something firmly.
* Comparative adjectives compare two things. To form the comparative of most adjectives, we add "-er" or "more" to the adjective.
* Since "tenacious" is a longer word, we use "more" to form the comparative: more tenacious.
For example:
* "He was more tenacious than his opponent and finally won the game."
* "The roots of this plant are more tenacious than those of the others."