Perhaps the most significant reason is because they typically require a large level of support from the ruling party. It's only going to support a change if the change benefits itself- if it helps keep it in power.
More representative systems typically leech power away from the currently dominant party- FPTP is not a very representative system, so any proposed move away from it would hurt the current party in power. It also has the mathematical property of exaggerating the difference in support for parties, so even a modest plurality of votes overall can lead to a massive majority of seats. So it's very rare that a government would countenance it because even a large majority might disappear at the next election under a better system. So this is a rare opportunity to improve how elections are conducted in the UK.
If AV doesn't get adopted, there may never be another chance to move to a different system in our lifetimes.
If it does get adopted, we may have to wait a while before further reform, but the case for it will be much stronger. If you prefer a different system to AV, your best chance of getting it at all is to vote for AV now.
Yes to AV because improvement has to start somewhere.