You are right, Inter Milan always has a big name and with a good history too, if compared to Napoli then we will find a lot of differences. Maybe some people will say that it is just history and what happens now could be different. That is not wrong, but one thing we can see is their journey too. When a team with a long journey they are always in tight competition and also win several titles, then mentally they will also be more trusted. This is a habit and in fact it happens like that.
Inter Milan have good history and one of most successful clubs in Serie A and recent seasons, they are most stable clubs in joining Scudetto competition and win it two times. This season they have to chase Napoli but with very close difference now, Inter Milan can take a first position anytime and they are strong as well as stable enough in performances to wait for that chance.
Not to underestimate Napoli, they are also a good team, but here we are just having an opinion.
Napoli won Scudetto two seasons ago, but the last season is like nightmare for them, and this season began without expectation that they can win Scudetto or just join Scudetto race. Antonio Conte is an important brick of development in Napoli, as he convinced Lukaku to join, and used this striker as well as Tominay very efficiently.
Napoli don't have as same good player resources like Inter Milan but with Conte, they have changed Scudetto competition and it is no longer dominated by Inter Milan.