Why might my tutors from several years ago not respond to my email request for a meeting?
The absence of a response can be because of several reasons. Your mail can indeed end up as spam but it is usually the e-mail system that sorts it there automatically, not the person. Sending additional mails will then be pointless unless the recipient actually goes through the spam folder to look for erroneously sorted mails.

A more likely reason for the lack of response is a matter of priority. I think etiquette would dictate at least a brief answer to the effect that the mail was received but cannot be followed up. You are requesting something that is a personal favour to you. As such, and as has been commented, it is only fair and indeed professional to provide a short and clear statement of the purpose of the meeting. After all to meet someone during work hours better be about something work related.

The tutors have probably also tutored many others. From their perspective, you, without any derogatory intent, will be one of many. I often get requests from students who apparently think that because they are out of the program they should be "friends" with me. I would end up with many such "friends" but none that would have any significance either professionally not privately. Hence there is an unbalance between how your tutor has meant something for you and what you have meant to the tutor. This is built into this sort of relationship. Hence, you need to look at the formal role of both parties and evaluate if your reason for contacting the tutors is mostly a professional issue or a private issue. In the latter case, the response may be less than what you would wish for. If your cause is professional, you are far more likely to get a timely and satisfactory reply.