There are two major reasons why śivalinga is worshipped more than the actual murti (form/vigrahaṃ) of Supreme Lord Parameśvarā.
In entire Hinduism (Sanathana dharma) Śaivam is the only ideology which exists in both the perspectives.
- Universal worshipping freedom.
- Vedic/Sāmpradāyik methods of worship.
Whereas other heteredox and Folkloric ideologies like Bauḍha, Jaina, Vaiśṇava, Shāktha etc: are subjected only to their respective domain like philosophical texts, agamas, tantras etc:
As I mentioned above, the two reasons why Lord Shiva is worshipped as Lingam is because:—
- Jyoṭirlingaṃ — As we are well aware, Sanātana Dharma's 12 oldest Temples are the 12 Jyotirlingams (Dwādaśa Jyotirlingam) which are essential root for the Śaivam. All the temples being in Lingam form is the main reason why Lingam-worship is more significant among the devotees.
- Vedic and Sāmpradāyik ideology — The Vedas (Shvetāśvatāra Upaniśad) clearly mention that Lord Shiva in his murti form in Dakshinamurthy (i.e South facing or Dakshina mukham), even ancient Vedic worshippers of Lord like Sureśvarā (disciple of Bhagavatpāda Śaṅkarachārya) worshipped Lord Shiva as Dakshinamurti alone in his humane form (check Manasollasa Vartika), apart from that they worshipped Chandramoulishvara which is a lingam form of Lord.
ajāta ity evaṃ kaścid bhīruḥ prapadyate / rudra yat dakṣiṇaṃ mukham tena māṃ pāhi nityam // 4.21 //
It is because you are birthless, that some rare souls, frightened by birth and death, take refuge in you. O Rudra, may your benign south facing form protect me for ever!
~ Shvetashvatara Upanishad 4.21
Shiva can be worshipped in any form (with respect to lingam) like:—
- Chandramoulishvara.
- Rameshvara.
- Omkareshwara.
- Mallikarjuna.
- Kedaralingam. Etc:
But if anyone tends to worship Lord in his humane form, they are permitted to worship or meditate on him in his Dakshinamurthi form (South facing benign form) which is in accordance with Vedic/Sāmpradāyik worshipping mode as explained by great Vedantins like Sureśvarā bhagavatpāda etc:
SrI Vishwanatharpanamastu ||