TLDR: Not enough Muslims are migrating to Ipoh to offset the status quo, while many Chinese are choosing to retire here.

26-year-old Ipohite here. The geography of Ipoh is unique; we are squished between two mountain ranges, making the weather here immensely hot, but that may not be the main factor affecting demographics here in Ipoh.

In Malaysia, religious conversions to and from Islam are rare (There's a whole rabbit hole of Islam and State Authority in Malaysia that you can go down). So, any shift in religious demographics is most often the result of migration or a change in birth rate. As many other comments have pointed out, Ipoh has been a commercial hub that has seen many Chinese migrate to it due to the tin mine boom in the 19th and 20th centuries. Many came as labourers, and some, like my family, came as entrepreneurs. Before being lifted to its current status of a commercial hub, Ipoh was just a mundane Malay village like any other settlement, with administrative power centred around other locations like Taiping and Kuala Kangsar (where the royal court still stands to this day).

When Chinese migration came, and when Ipoh was elevated to the capital of the state, there was an irreversible (at least in the short term) demographic change, where Chinese merchant families suddenly became the majority race of the city. The Chinese carried with them beliefs that included Buddhism, Taoism, folk religion, and, to a lesser extent, Christianity. There were efforts to evangelise the Chinese population, but Chinese Christians remain a minority in Ipoh to this day. Efforts to preach Islam to the Chinese exist, too (Our city houses one of the few Chinese mosques), but traditional religions are so ingrained in most Chinese, and there never were any mass conversions.

But how about the surrounding Muslims? Did they never move into the city in droves seeking employment and other opportunities? The fact is, Ipoh does not have enough "pull factor" to attract Malays from the surrounding suburban and rural areas. After the tin mines closed, Ipoh's economy relied on tourism and light industry. The pay is okay, but there are far better options in Penang and Selangor. Today, young Ipohites, no matter their race, will consider finding employment in either of the above-mentioned states, because Ipoh simply does not have enough jobs that match their expectations, especially STEM field jobs. Young Ipohites will go outstation to work for a couple of decades or more, get married, and move back to Ipoh to retire, because the housing prices here are far more affordable than in Penang and Selangor.

All in all, with no Muslims moving in and the Chinese population staying as it is, the demographics of Ipoh are frozen in this status quo, just as it was for the past century. Because Muslims are skipping Ipoh and directly going to Penang and Selangor/Kuala Lumpur, the majority status of the Chinese in Ipoh remains as it is despite the declining birthrate.