Not replying to messages can be disrespectful in some cases or it may be simply that the person is busy or cannot respond right now or they're thinking about what to say.
Some people take so long to reply for a variety of different reasons which include being busy, forgetting about the message, forgetting to reply, are thinking about how they want to respond, feeling anxious about what to say or they may have not seen the message yet.
Someone who does not reply may also be experiencing problems with their network or phone or are in a situation in which they cannot respond.
In some cases not responding to you or a message can be a signal of disinterest or a lack of respect, especially in personal and professional contexts.
The person who does not respond may be working, in a meeting or doing other activities or even sleeping which prevent them from checking their messages.
The person could also be in an area where no cell service is or they have a dead phone battery or forgot to pay their phone bill.
The person not responding may have seen the message but forgot to reply or they replied and the message didn't go through or they have paused responding back while thinking about a reply.
If the person does not reply back within a few days then they may be ignoring you and if they don't respond to a new message after a few days then it could be a sign that they are avoiding you or they don't want to respond back to you.