This is perhaps because Estragon has far more to worry about. Of the two, Estragon suffers the most physically. He is kicked by Lucky, takes numerous pratfalls, and is beaten by a gang of thugs every night. He also has great difficulty with his boots. In the first act, they do not fit and cause him pain. At the end of the act, he leaves them on the side of the road in disgust. In the second act, he finds them in the same place, now mysteriously fitting him. This boot focus (in terms of the character being foot centred) may be interpreted as being representative of his lower status (see Vladimir's hat) and his "earthy" nature (e.g. his love of carrots, and, secondly, radishes, etc.).
Estragon has a misanthropic view of humanity: he considers people to be "ignorant apes" and seems to want them to leave him alone. However, he is very attached to Vladimir (despite occasionally bickering with him): he needs protection, and Vladimir provides it. Estragon is often seen as the child to Vladimir's adult, and as such looks for parental security in him.
He should not, however, be written off as merely a childish simpleton. He easily matches Vladimir in verbal melee (he delivers the ultimate insult in calling Vladimir a "Critic!"). He also shows an artistic side, and even claims to have once been a poet. His brief, but evocative, monologue about the "maps of the Holy Land" is very poetic in nature. Later, he purposely misquotes a Shelley poem, "To the Moon":
Art thou pale for weariness
Of climbing heaven and gazing on the earth,
Among the stars that have a different birth -
And ever changing, like a joyless eye
That finds no object worth its constancy?
Estragon's play on it:
"Pale for weariness...Of climbing heaven and gazing on the likes of us."
Many famous actors have taken on the role (especially those of the afformentioned body type), notably Bert Lahr and Robin Williams (with Steve Martin as Vladimir). Beckett is even said to have wanted Marlon Brando in one production (with Buster Keaton as Vladimir).