One cornerstone of Chinesew foreign policy is that a country’s internal affairs are that country’s internal affair. The only exception I can think of in the past 50 years is the Chinese acquiesence for the US led invasion of Afganistan.
China sees meddling in a country’s ‘internal’ affairs as a slippery slope. China could itself come under attack for policies in tibet, inner mongolia, manchuria, xinjiang and border disputes with russia, india, philippines, viet nam, malaysia, indonesia, etc.
personal gain from Iran specifically (access to oil) may be a factor but the chinese have always supported non-interference in sovereignity issues.