For many Pro-Brexit people they felt May's deal didn't really leave the EU. A very major sticking point is that provisions in the exit agreement, refereed to as the "backstop", that in theory and possibly in practice could legally trap the UK in a half completed transition deal forever with the UK always subject to EU rules. Overall the agreement May put forward is basically an agreement to start the next phase of negotiations to start discussions on a transition agreement which would maybe hopefully be finished by the year 2020 or 2021 or a day after never. Also paying 39 billion pounds and not knowing what the next phase of negotiations would deliver.

For Remainers(Anti-Brexit) the agreement was to start the process to leave the EU so of course those people didn't like the agreement either.