I am no definite expert on the topic but this customization of a flight, called ancillary services, are usually marketed to travel agents as a profit generator.
First let's clarify the different actors. The airlines offer and operate flights, the GDS (Global Distribution System) companies distribute the flights to travel agents and the latter sell to the traveller (you). The travel agents can be your local travel agent in his office, the online travel agent (OTA) like Expedia, Opodo or Kayak, or the airline itself through its website, phone service or offices.
In all cases the travel agent decides which ancillary service to sell to the customer. The travel agent probably pays to implement the service distribution (to the GDS) and receives a fee per transaction made. As meal selection is not generally a lucrative option - it is usually free of charge and I think religious or allergic people would react badly if they had to pay to get a specific meal - it is not worth offering for the travel agents. As @drat commented, the airlines have also little incentives (and higher logistic costs) to offer them. So it makes sense not to offer meal choice directly online. Note that some OTAs offer a free text field at the end of the booking without promising it will be read by anyone (I have seen that on govoyages, for example).
In the case of Emirates and Singapore Airlines, I have in mind that their image is based on open-mindedness, globalization and respect of the culture of the travellers. I insist on the fact that it is the image I have, you might not feel the same. But offering a choice of meal is a good marketing argument for a world-wide growing airline. On the opposite, Air Transat mostly targets Canadians visiting the Caribbean and transatlantic travellers, with less of a variety of travellers. Offering a large variety of meals would be more a burden than a marketing argument to them. And Air Transat offers ancillary services on their website like seat selection, so it is a deliberate choice not to offer meal selection.
In the end, I suppose that the reasons are economical. It costs less to offer less choice and not to prepare too many meals. It depends on the travel agent you buy your tickets from, but there is no rule for who offers meal selection or does not. If it matters a lot, you can try to buy your ticket from another travel agent, it might work better.