Why did not Karna use Shakti given by Indra on the 14th day of battle when Karna was badly beaten by Arjuna?

 

That is called the power of Fate - Karma that governs the World.

a) After the death of Saindhava, when Duryodhana was accusing Drona, Karna stops him stating that humans, however powerful they may be, are just puppets in the hands of Fate - Karma.

दैवादिष्टोऽन्यथाभावो न मन्ये विद्यते क्वचित्॥

यतो नो युध्यमानानां परं शक्त्या सुयोधन। सैन्धवो निहतो युद्धे दैवमत्र परं स्मृतम्॥

परं यत्नं कुर्वतां च त्वया सार्धं रणाजिरे। हत्वाऽस्माकं पौरुषं वै दैवं पश्चात्करोति नः॥

सततं चेष्टमानानां निकृत्या विक्रमेण च ।

दैवोपसृष्टपुरुषो यत्कर्म कुरुते क्वचित्। कृतं कृतं हि तत्तस्यट दैवेन विनिपात्यते॥

यत्कर्तव्यं मनुष्येण व्यवसायवता सदा। तत्कार्यमविशङ्केन सिद्धिर्दैवे प्रतिष्ठिता॥

I think that which is ordained by Fate never occurs otherwise. And since, O Suyodhana, in spite of ourselves fighting to the utmost extent of our powers, the ruler of the Sindhus has been slain in battle, it seems that Fate is all-powerful.

With thyself we had all been exerting to the utmost of our might on the field of battle. Fate, however, baffling our exertions, did not smile on us. We have always exerted to injure the Pandavas, relying both on deceit and prowess.

Whatever act, O king, a person afflicted by Fate does, is frustrated by Fate, however, much the person himself may strive to achieve it. Whatever, indeed, a man endued with perseverance should do, ought to be done fearlessly. Success depends on Fate!

According to my understanding, the Fate diverted Karna’s mind from using Shakti, given by Indra, on Arjuna on the 14th day of War.

b) After the death of Ghatotkacha, Sanjaya says to Dhritarashtra that Fate is the main reason for Karna’s not using Shakti on Arjuna.

संग्रामाद विनिवृत्तानां सर्वेषां यॊ विशां पते
रात्रौ कुरु कुलश्रेष्ठ मन्त्रॊ ऽयं समजायत

परभातमात्रे शवॊभूते केशवायार्जुनाय वा
शक्तिर एष विमॊक्तव्या कर्ण कर्णेति नित्यशः

ततः परभातसमये राजन कर्णस्य दैवतैः
अन्येषां चैव यॊधानां सा बुद्धिर नश्यते पुनः

दैवम एव परं मन्ये यत कर्णॊ हस्तसंस्थया
न जघान रणे पार्थं कृषणं वा देवकी सुतम

Sanjaya said, 'Returning from battle every day, O monarch, all of us, O foremost one of Kuru's race, used to debate in the night and say unto Karna. Tomorrow morning, O Karna, this dart should be hurled at either Kesava or Arjuna.'

When, however, the morning came, O king, through destiny, both Karna and the other warriors forgot that resolution. I think destiny to be supreme, since Karna, with that dart in his hands, did not slay in battle either Partha or Devaki's son, Krishna.

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c) Further, on the 14th day itself, Shri Krishna stops Arjuna, after the death of Saindhava, from facing Karna. Shri Krishna reminds Arjuna about Shakti given by Indra to Karna.

अलम एष महाबाहुः कर्णायैकॊ हि पाण्डव
किं पुनर दरौपदेयाभ्यां सहितः साततर्षभः

न च तावत कषमः पार्थ कर्णेन तव संगरः
परज्वलन्ती महॊल्केव तिष्ठत्य अस्य हि वासवी
तवदर्थं पूज्यमानैषा रक्ष्यते परवीरहन

अतः कर्णः परयात्व अत्र सात्वतस्य यथातथा
अहं जञास्यामि कौरव्य कालम अस्य दुरात्मनः

Thus addressed by Savyasachin, the mighty-armed Kesava, endued with great energy, replied in these opportune words,

'The mighty-armed Satyaki is singly a match for Karna, O son of Pandu! How much superior then will this bull among the Satwatas be when he is united with the two sons of Drupada!

For the present, O Partha, it is not proper for thee to fight with Karna. The latter hath with him the blazing dart, like a fierce meteor, that Vasava gave him.

O slayer of hostile heroes, he hath kept it for thy sake, worshipping it with reverence. Let Karna then freely proceed against the Satwata hero.

I know, O son of Kunti, this wicked wight's hour, when, indeed, thou wilt, with keen shafts, throw him down from his car.'