Why were the asuras more interested in doing penance to obtain the throne of the Ruler of Heaven compared to that of Lord Brahma, when it can be understood that the post of Lord Brahma wielded more power compared to that of Lord Indra.
Well, faith and belief are the only thing that divides an āstika from a nāstika.
If you believe than all are the menifestations of the Nirguna Para Brahman and are themselves Saguna Para Brahman.
yo ‘sṛjad dakṣiṇād aṅgād brahmāṇaṃ lokasaṃbhavam | vāmapārśvāt tathā viṣṇuṃ lokarakṣārtham īśvaraḥ | yugānte caiva saṃprāpte rudram aṅgāt sṛjat prabhuḥ |” (MBH 13:14:183).
“Thou (Maheshwara) art he that hadst created from thy right side the Grandsire Brahma, the Creator of all things. Thou art he that hadst created from thy left side Vishnu for protecting the Creation. Thou art that puissant Lord who didst create Rudra (the destroyer deity Kalagni-Rudra) when the end of the Yuga came and when the Creation was once more to be dissolved”.
"Vayu said, I shall narrate how Rudra was born and how Brahma and Vishnu were born from each other. The three are Atmans as causes of born of Mahesvara; they are cause of creation, sustenance and annihilation of the Universe consisting of mobile and immobile beings. Endowed with great qualities they are sanctified by great Lord. Presided over by his Shakti, they can perform their activities. Brahma can create, Vishnu can protect and Rudra can annihilate. But they rivaled with one other. Desirous of excelling one other they propitiated their father the Supreme Lord Maheshwara by means of penance. Attaining all around favor of Lord at outset in Previous Kalpa, Rudra created Brahma and Vishnu. In another Kalpa, Brahma created Vishnu and Rudra. In another Kalpa, Vishnu created Brahma and Rudra. Thus in different Kalpas, Brahma, Vishnu and Rudra desiring mutual benefit are born of one another. Based on the events in their respective Kalpas, their process in being the cause of origin of one other is extolled by Sages. (Chapter 13, Vayavivasamhita, Shiva Purana)."
For Lord Indra you can see this answer of mine..
And, if you don't believe then they are all just a mere post.
ब्रह्मत्वं केशवत्वं वा शक्रत्वं वा सुरैः सह। त्रैलोक्यस्याधिपत्यं वा तुष्टो रुद्रः प्रयच्छति 13-49-65
"Vasudeva said,...If Rudra be gratified with a person, he can confer upon him the states of either Brahma or of Kesava (Vishnu) or of Sakra with all the deities under him, or the sovereignty of the three worlds. Those men, O sire, who worship Bhava even mentally, succeed in freeing themselves from all sins and attain to a residence in heaven with all the gods..."
भगवानुवाच, शैलादे – – – – ६- तव तुष्टोस्मि साम्बिकः । यं त्वद्य वृणुषे कामं सर्वं तं प्रददामि ते ।।१३ ।। ब्रह्मत्वमथ७ विष्णुत्वमिन्द्रत्वमथवान्यताम्८ । आदित्यो भव रुद्रो वा ब्रूहि किं करवाणि ते ।।१४।।
Pleased with Nandi’s devotion, Shiva offers him boons: Ask whether you want Brahmatva, Vishnutva, Indratva, or something else – Aditya or Rudra.
न मे देवाधिपत्येन ब्रह्मत्वेनापि वा पुनः ।।४६।। न विष्णुत्वेन देवेश न इन्द्रत्वेन भूतप । इच्छाम्यहं तवेशान गणत्वं नित्यमव्ययम् ।।४७। । नित्यं२७ त्वां सगणं साम्बं प्रसन्नं सपरिच्छदम् । द्रष्टुमिच्छामि देवेश एष मे दीयतां वरः ।।४८।।
Nandi replied.: I don’t want Brahmatva, Vishnutva, Indratva, I want to be near you as one of your servants.
I hope this clarifies all your queries. Prd..